From "Lint Catcher" to "Cornonation", he lives on Mewni with Star and her family. As of "Doop-Doop", he goes back to living on Earth with his family. From "Here to Help" onward, he is Star's boyfriend.
Marco is a Latino-American teenager of medium height with a slender build, tan skin, dark brown hair, brown eyes, and a mole on his right cheek. He usually wears a light gray shirt, a red hoodie, dark gray skinny jeans, and olive and white sneakers.
As an adult in Hekapoo’s dimension, he is taller and muscular, with shaven facial hair, multiple piercings in his right ear, and a scar on his left eye. As a result of Hekapoo's constant searing smacks to the back of his head, he also has a bald spot.
In "Deep Dive", Marco has crescent moon-shaped emblems on his cheeks while casting the All-Seeing Eye spell. These cheek marks appear again in "Cleaved" while he hugs Star in the collapsing Realm of Magic after all magic is destroyed.
Marco is kind, responsible, very organized, and supportive of others. He has a reputation at his school for being the "safe kid" due to avoiding danger and being cautious, though he insists that he is a "misunderstood bad boy" and lives for danger. He prefers keeping his possessions neatly organized. Marco is smart, gets good grades in his math and psychology classes, and figures out how to find a replacement for Star's wand charger using the metric system. Because he likes it when things go his way, he can become unstable when they don't, like when he snaps at Star for ruining his room and insanely trying to live in front of Stop & Slurp. Although he isn't used to the kind of craziness that Star brings, he admits that he would like a little danger in his life and finds fun in fighting monsters with karate.
He often struggles with self-confidence, afraid of being ridiculed by others, and can become nervous easily. "Naysaya" reveals that he has many insecurities and emotional hang-ups. He has an occasional temper and can get frustrated and/or annoyed with others, such as with Star when she intrudes upon his life, or recklessly drags him into overly dangerous situations. However, he soon makes amends with those who cause him grief, like thanking Pony Head for saving him in "Party With a Pony". He cares deeply for those he is close to, attempting to save Ferguson from being kidnapped in "School Spirit" and trying to cheer up Star whenever she is feeling upset. Marco also has an urge to occasionally protect Star and keep her safe, as depicted in "Sleep Spells" and "Blood Moon Ball".
Over the course of the series, Marco becomes less overly cautious and more confident, especially in his romantic pursuit of Jackie Lynn Thomas in "Freeze Day" and "Naysaya". He progressively takes more risks as a result of his friendship with Star, but he sometimes still struggles with self-confidence. After Marco moves into the Butterfly Castle, he usually tends to prioritize his wants over his responsibilities which can cause him to mess up his important duties such as him wanting a job higher up so that he could see Star or keeping Star's sleep-portaling a secret from Hekapoo so that they can continue to work together and go on adventures. Whenever he realizes this, he does his best to make things right.
In his role as Star's squire, Marco takes his responsibilities seriously and carries them out as best he can, as shown in "Trial by Squire", "Is Another Mystery", and "Divide". However, his intense focus sometimes causes him to ignore other people in his life, such as his parents in "Marco Jr."
Marco cherishes her company and soon comes to see her as his best friend. He thinks her skills with fighting monsters are impressive, and he likes joining her on her adventures. With Star's help and adventurous ways, Marco is learning how to be less cautious and instead make the most of his life. In turn, he makes sure that they do not get into too much trouble. If she is upset, like in "Cheer Up, Star", he does what he can to make her happy and feel better.
While Star's bubbly, helpful, and reckless personality can sometimes cause Marco a lot of trouble, irritate him, or even cause him physical pain, he appreciates having her around, even calling her the coolest girl he knows. He can be a little overprotective of her at times and underestimates her ability to handle things on her own, which often infuriates her. "Sleep Spells" also shows that he sometimes feels inadequate compared to her, as he is just a normal kid, and Star is a magical princess. Despite this, he is willing to help her where required and they are usually fast to make amends whenever they end up in a fight or disagreement.
Ultimately, their friendship is steadfast and unbreakable; in "Gift of the Card", when the two were facing imminent death by the gift card Star had gotten him, Marco told Star that they should hug, so whoever found their "charred skeletons" would know they were best friends.
In "Blood Moon Ball", Star and Marco briefly dance together under the light of the Blood Moon, which according to legend binds their souls together for eternity.
In "Storm the Castle", despite the argument that they have after returning home, Marco shows no sign of anger or annoyance when Star comes to rescue him, and they hug after Star throws Ludo into a portal.
In "Face the Music", Marco learns that Star has a crush on him, causing constant awkwardness between them in "Starcrushed". At the encouragement of his father, Marco talks with Star about these feelings, which she initially denies, and they manage to work through the strain in their relationship and remain best friends. After Star finds out she has to return to Mewni, she confesses her feelings to Marco and runs away before he can respond, leaving their relationship uncertain.
In "Return to Mewni", Marco is shown to have fallen into a depression following Star's departure. In "Marco and the King", he visits Butterfly Castle to see Star and give her a box of Captain Blanche's Sugar Seeds (her favorite cereal), and he asks her father if she is in any danger since he left Earth in a big rush. When Marco later rescues Star from a castle dungeon, they reunite with a hug, indicating they care a great deal about each other regardless of the circumstances of their separation in "Starcrushed". Near the end of "Toffee", when it appears that Toffee has killed Star, Marco attacks him in tears by punching a hole in him, which indicates how much he has grown to care for Star.
In "Scent of a Hoodie", when Star doesn't have a reason to go back to Earth and stays on Mewni, he and Star share a tearful goodbye hug before they part ways. He later receives his missing hoodie at the end of the episode and, after noticing that it smells like Star, he sniffs it.
Following his return to Earth in "Sophomore Slump", Marco begins constantly talking about his experiences on Mewni, which Jackie interprets as him having developed feelings for Star. At her encouragement, Marco returns to Mewni in the hopes of becoming a knight and spending more time with Star. In "Lint Catcher", Marco is appointed as Star's royal squire so that they can hang out again.
In "Lava Lake Beach", Tad makes Marco finally realize that his feelings for Star are romantic. Though he initially tries to deny it, after seeing Star and Tom kiss, he becomes depressed and has no choice but to accept they are true. In "Deep Dive", his concern for Star is so great that he risks wielding her wand, and he blushes when Star hugs him in the end.
In "Stump Day", Marco throws a surprise birthday party for Star on Stump Day; though she is initially conflicted due to her devotion to the Stump, she later thanks him for the gesture.
In "Marco Jr.", Star is shown to be the only one who can properly fill out Marco's essence test in his stead, showing that, due to their interdimensional travels, she knows him better than his own parents.In "Booth Buddies", Marco works through his feelings for Star and how much their relationship has changed since the end of "Starcrushed". This results in him confessing his feelings for Star, much to her surprise. In a moment of emotion, they kiss each other, leaving them both in a state of complete shock and regret, and both attempt to forget the moment ever happened.
In "Divide", Marco tries to comfort Star after she is declared acting queen of Mewni and the pressures of responsibility get the better of her. When they make a plan to find Moon and keep Meteora distracted, they confide in each other how scared they are and share a tender hug, also blushing when trying to hide such affection. At the end of "Conquer", when Marco and the rest of Meteora's victims regain their souls and return to normal, he and Star share another embrace.
In "Curse of the Blood Moon", Marco concludes that he still likes Star to the point of unnaturally liking every single thing about her, and with Eclipsa and Janna's guidance decides to re-confess his feelings. Learning that Star has feelings for him as well, they wonder if their budding feelings for each other are only the result of their souls being bound at the Blood Moon Ball. With Tom and Relicor Lucitor's help, they have that bond severed, and they lose their memory of that evening. However, during the soul-severing process, they wonder if their feelings for each other were actually not the Blood Moon's doing.
In "The Knight Shift", when Marco reveals that he plans to go back to Earth someday to be with his family and go to college, Star is sad at first, but the two agree to never stop having adventures together when that happens, and he gives her a cape like the one she made for him.
In "Britta's Tacos", several of Marco's friends on Earth mistakenly believe that he and Star bore a child together (actually Marco's baby sister Mariposa). Marco is also encouraged by Jackie, and later Tom in "Jannanigans", to start a relationship with Star and to not mess it up. In the latter case, though Marco still tries to deny his feelings, Tom remains convinced that their bond is stronger than that.
In "Mama Star", Marco tells Star he loves her while under the memory-erasing influence of the Realm of Magic. In "Here to Help", Star talks to Marco about this, much to his mortification. Marco reveals he's had feelings for Star since the very beginning that slowly grew into love and he tried to suppress it for the sake of their friendship, only for Star to confess her love for him as well. They kiss and begin a romantic relationship, with Star referring to Marco as her boyfriend in "The Tavern at the End of the Multiverse".
In "Cleaved", despite his initial misgivings, Marco eagerly goes along with Star's plan to destroy all magic. After they succeed in doing so, Star and Marco hug even as the realm collapses around them, believing they belong together with or without magic. The remaining magic within them creates a dimensional rift; when the rift explodes, the dimensions of Earth and Mewni merge into one, allowing Star and Marco to be together.
Rafael and Angie DiazMarco loves his parents, but he is sometimes annoyed by their cheery and optimistic personalities. He can also be embarrassed by his mother, such as when she said the "Ph.D" in "Doctor Marco, Ph.D" stood for "pretty handsome dude". In "Starcrushed", Marco takes his father's advice of talking with Star about their situation. In "Heinous", he is annoyed when they take Miss Heinous' side during her home invasion. In "Sophomore Slump", Angie tries to help him readjust to Earth life by recalling how she became arrogant and condescending to others after a summer in France.
When Marco prepares to depart for Mewni, Rafael shows concern, but Angie declares that Marco needs to go. In "Marco Jr.", his parents are upset with him for not visiting often. Upon finding out that they don't know as much as they thought about their son, Rafael and Angie are deeply saddened. After Star restores Marco's distorted appearance, the family resolves to spend more time together. In "Lake House Fever", Star states that Marco is making dinner for his parents. In "Doop-Doop", Marco moves back to Earth to be with his parents as well as his new baby sister Mariposa.since kindergarten and hopes that she will like him back in the same way someday. However, he has a hard time getting her attention without messing up or losing his cool. He is also overly nervous around her, not even picking up the courage to say "hello" to her until recently. More recently, he is starting to show some progress in getting closer to her in spite of his nervousness. In "Sleepover", Marco confesses his feelings to Jackie, but he admits he is not sure whether he likes her or the image he has of her in his head. Despite this, he is willing to admit all of his insecurities to her in order to secure a date with her in "Naysaya".
In "Bon Bon the Birthday Clown", Marco goes on his first date with Jackie, on which he gets to know her better, and the two have such a great time together that they share their first kiss. It is stated in "Just Friends" that the two are officially a couple.
In "Starcrushed", the two share some casual boyfriend-girlfriend interaction, hanging out together and feeding each other pizza nuggets. At one point, Jackie asks Marco to try and read her mind, and she rewards his guess with a kiss on the cheek.
In "Sophomore Slump", Marco's adventures on Mewni distance him from his friends and family on Earth, and Jackie breaks off their relationship before it causes them long-term misery.
In "Britta's Tacos", Marco and Jackie meet up again after some time since their breakup. Marco previously had been anxious about meeting Jackie, believing she would be bitter over their relationship and breakup. However, the two agree they weren't right for each other, and decide to start over as good friends. In "Cleaved", Jackie lends Marco her skateboard so that he can reunite with Star.football team, Marco fears that the rival team, the Warriors, will kidnap him, so he does everything he can to keep his friend safe, despite Ferguson's protests. Marco cares about his two friends quite a lot, and became devastated after believing Ferguson had died due to one of Star's landmines. Marco and Alfonzo's relationship has yet to be seen in great detail, but it can be assumed that it is similar to Marco and Ferguson's. In "Sophomore Slump", Marco briefly gets into a fight with them due to his constant talking about Mewni, but they reconcile shortly before Marco leaves Earth. In "Britta's Tacos", Marco reunites with Alfonzo and Ferguson, who are overjoyed to see him. Amethyst Arcade, but she eventually comes back and saves him from the guards from St. Olga's Reform School for Wayward Princesses, even though it meant she would have to go back with them. Marco greatly appreciates her saving him, and ends up on better terms with her, wishing her good luck in "princess jail" before she is taken away.
Later on, he still holds unfavorable feelings towards Pony Head, but begrudgingly joins Star in breaking her out of St. Olga's for her birthday. After they manage to escape, Pony Head congratulates Marco for inciting a riot amongst the other princesses, with Marco accepting the compliment. In all future encounters, the two are far more civil with each other, their initial animosity all but forgotten.
However, Pony Head's wild, reckless behavior still irritates Marco on occasion. In "Pizza Thing", Marco begrudgingly includes Pony Head in his and Star's tradition of "Friendship Thursday", but his strait-laced personality clashes with her spontaneity and partying lifestyle. After receiving some words of wisdom from a little boy, however, the two manage to put aside their differences and declare a truce.
In "The Bounce Lounge", he expresses disinterest when Pony Head arrives at the Diaz Household in tears, believing she is crying over something trivial, and he gets annoyed when she praises Milly Sparkles for repeating his words of wisdom. "Monster Bash" shows that they still have somewhat of a rivalry over being Star's best friend. In "Divide" and "Conquer", Marco includes Pony Head as one of his "Marc-nificent Seven". In "The Ponyhead Show!", he is excited about being featured on Pony Head's show, but he gets annoyed and disappointed when she makes it all about her.Red Belt", bowing to him when instructed and stating that he wants to someday be like him. Unfortunately, he loses respect for his sensei when it is revealed he still lives with his mother and shows no interest in getting his red belt. By the end of the episode, however, Marco's sensei regains his respect after his nonsensical teachings help them both earn their red belts.
In "All Belts are Off", Marco is upset when his sensei picks Jeremy Birnbaum to represent his dojo instead of him, but he is later pleased to learn that his sensei considers him a friend and "dojo bro". In "Sophomore Slump", Marco accidentally upsets him by suggesting he's earned more than his sensei has, but they reconcile shortly before Marco returns to Mewni.Monster Arm", he wants to beat Jeremy in a tournament and not be made fun of for losing or "wussing out". When Monster Arm later attempts to slaughter Jeremy, Marco is mortified and intervenes. Outside of karate, Jeremy continues to bully Marco on a regular basis, much to Marco's frustration, as revealed in "Cheer Up, Star". Marco tends to rebuff Jeremy verbally, apparently not wanting to use force outside of karate matches. However, he chases after Jeremy in "The Banagic Incident" after Jeremy pushes him too far. Mewberty", they start off on less than good terms when Glossaryck mistakes him for a girl and refuses to help him. Marco dislikes Glossaryck's intentionally roundabout helping methods and overall mischief, demonstrated further in "My New Wand!", "Star on Wheels", and "By the Book", but Glossaryck does tolerate Marco and enjoys messing with him. When Glossaryck appears to betray Star in "Raid the Cave", Marco remarks that he never thought of him as much of a team player. When Marco returns to Mewni in season 3, he helps Star take care of the seemingly mentally-impaired Glossaryck, notably in "Sweet Dreams", "Night Life" and "Is Another Mystery". Mr. Candle Cares", Marco and Tom manage to come to an understanding, with Marco convincing Tom to let Star go.
In "Friendenemies", Tom invites Marco to hang out with him. Over the course of the night, Marco bonds with Tom, mostly over their shared love of the band Love Sentence. When Marco learns that Tom was only hanging out with him to complete his anger management course, he leaves in disgust. Tom makes him a peace offering by resurrecting deceased martial artist Mackie Hand, and they watch him beat up some movie ushers together, though they end the episode stating they still do not like each other. Despite this, they have hung out semi-regularly since then, as Tom says in "Naysaya".
In "Lint Catcher", they are happy to see each other when Marco returns to Mewni. In "Stump Day", Tom and Marco get into a disagreement over Marco's idea to throw Star a surprise birthday party. Their fight escalates when Marco accuses Tom of being a bad boyfriend to Star, but they eventually reconcile.
In "Is Another Mystery", Marco fakes losing Glossaryck so that Tom can go investigate Buff Frog's disappearance with Star instead of him, showing that, despite any feelings he may be harboring for Star, Marco was willing to give up time with her for Tom.
Marco and Tom work together a great deal in "Divide" and "Conquer". When the original plan to trap Meteora fails, Tom stands up for Marco, stating that he believes in his value as a leader and appreciates his stubborn nature. When only the two of them remain, Marco admits that he kissed Star in a last ditch attempt to have Tom retreat and save himself and Star. Tom originally believes it to be a bluff and appears relatively unfazed at first, leading Marco to have to tell him it actually happened. Nonetheless, they are both willing to give their lives for each other, showing how much their bond has grown.
In "Swim Suit", they work together to make a beach day for Star after she is forced to deal with a conflict between Rhombulus and Eclipsa. In "A Boy and His DC-700XE", Tom buys a dragon-cycle in order to join Marco's biker gang because he thinks Marco is cool. In "Jannanigans", Tom gives Marco his blessing to pursue a relationship with Star after they break up in "Sad Teen Hotline", showing how much he's grown to respect Marco and his friendship with Star. In "Cleaved", Marco refuses to give up on saving Tom from the purple unicorn and restoring his memories.Gift of the Card" or keeping things in a hidden compartment behind his locker in "Naysaya"). Nonetheless, he is willing to hang out with her on occasion and generally friendly with her when she isn't messing with him. In "Deep Dive", it is revealed that Janna had, at some point, hypnotized Marco into passing out by snapping her fingers and saying the word "chickenbutt". In "Cleaved", Janna considers Marco to be her friend, and she helps him reunite with Star. Running with Scissors", Marco is initially very annoyed by Hekapoo's condescension toward him, impishness, and tendency to smack and burn the back of his head. After sixteen years of hunting her and her clones down, he grows more tolerant of her antics, giving her the nickname "H-Poo", and their adventure together ends on more or less friendly terms. However, she still occasionally annoys him with her teasing.
In "Night Life", Marco helps Hekapoo deal with rogue interdimensional portals. When she finds out that they were being created by Star in her sleep, Hekapoo becomes angry with Marco for not telling her. While she agrees not to tell the rest of the Magic High Commission, she says that they are done working together. Despite this, he still includes her as part of his "Marc-nificent Seven" in "Divide" and "Conquer".
In "Cleaved", Marco expresses concern for Hekapoo after realizing that destroying magic will cause her to disappear. However, Hekapoo reassures Marco that she always felt magic needed to be erased, giving him one last slap before he departs, and he admits he's going to miss her.Goblin Dogs", he is initially put off by her lack of response to his conversation and keeping food to herself without offering him any. Regardless, Marco is able to confide in Kelly about his problems with Star and Pony Head, and he says it was nice talking to her at the end of the episode.
In "Lava Lake Beach", Marco comforts Kelly after her most recent breakup with her boyfriend Tad, and she likewise helps him through the heartache of seeing Star and Tom together. As they watch the Underworld Soulrise together, Marco offers Kelly his hoodie to keep warm, and she wishes him a happy birthday. In "Monster Bash", Marco is annoyed when the party guests imitate his karate poses as dance moves until Kelly asks to dance with him. They also dance together at the end of "Stump Day" and attend Ruberiot and Foolduke's wedding together in "Booth Buddies".In "Divide" and "Conquer", Marco includes Kelly as one of his "Marc-nificent Seven". In "The Ponyhead Show!", Marco and Kelly are co-hosts in the "Cooking with Earth Turd" segment on Pony Head's talk show. Marco accidentally upsets her when he says wrestling shows are fake, but she forgives him quickly, and later invites Marco to her house to cook Earth food.
In "Kelly's World", Marco visits Kelly's dimension for the first time and tells her he has fun whenever he's with her. After teaming up against a ruthless librarian, Marco reveals that he wishes he didn't have feelings for Star because they're getting in the way of the feelings he's starting to have for Kelly. The two agree to be "break-up buddies" to help each other get over their old feelings and move on, and Kelly kisses Marco on the cheek.
In "Cornball!", Marco and Kelly are co-commentators for the Mewman/monster cornball game. As of "A Boy and His DC-700XE", the two are no longer breakup buddies after a supposedly amicable separation. Though Kelly claims it was all on Marco and that only he was happy with the situation, they still seem to be on good terms.
Moon and River ButterflyAs the chosen guide for Star, Marco soon becomes acquainted with her parents. Marco does not interact often with Moon, but appears to respect her position, and in turn Moon appears to care for Marco to the point where she was only slightly annoyed after he took the Royal Magic Wand from her in "Cleaved".
Like Star, Marco has a closer friendship with River, to the point of being one of the few people to call River by his first name on a regular basis. After establishing a friendship in "Diaz Family Vacation" and "Camping Trip", the two end up becoming closer during the events of The Battle for Mewni, after Marco helps River to develop confidence and leadership skills without Moon's guidance in "Marco and the King", and after the two become cellmates for days on end in "King Ludo".
River's respect for Marco eventually grows to where he gives Marco a knight cape (actually his meat blanket) as gratitude and says he is worthy of being a knight himself, although this is under the guise that he would never see Marco again, and also unintentionally leads Marco to believe he actually is a knight. Along with Star, the two are later seen questing together in "Butterfly Follies" and "Escape from the Pie Folk".Divide" in order to draw her away from Butterfly Castle.
After Meteora is turned into an infant, her hatred of Marco continues to the point where even mentioning Marco's name is enough to make her angry, leaving Marco anxious over whether or not she retained her memories. Marco, for his part, is able to mostly put his former hatred for her aside, despite continuing to be scared in her presence. In spite of this, "Gone Baby Gone" reveals that, in addition to Meteora not retaining her earlier memories, Marco is actually Eclipsa's favorite babysitter for Meteora.
When Marco first meets Eclipsa in "Sweet Dreams", he is hostile towards her, due to hearing about her reputation as the "Queen of Darkness". This quickly turns to annoyance when she teases him for his karate stance (calling it a "sword hand dance"), and calls him "adorable". He also shows concern when Star relinquishes the wand to Eclipsa in "Conquer".
After Star and Marco join Eclipsa at the Monster Temple, Marco becomes far friendlier towards her. He also considers Eclipsa a confidante, talking with her about his feelings for Star in "Curse of the Blood Moon". In return, Eclipsa proudly knights him in "The Knight Shift", and considers Marco her favorite babysitter for Meteora.
Powers and abilities
- Karate: Marco is a student in the Tang Soo Do discipline of karate, promoted from green belt to red belt in "Red Belt". He is very capable of using karate chops, kicks, and punches to battle monsters as effectively as Star. He is able to use karate with potentially lethal force as demonstrated when he cuts off Tom's hand with a karate chop and punches a hole through Toffee's chest. In "The Knight Shift", Marco defeats four knights, including experienced knight Sir Stabby, with minimal effort - using only his feet.
- Culinary prowess: Marco is good at making nachos. In "Pizza Thing", he makes a pizza with Pony Head, which had too many mushrooms for Star's liking. In "Baby", Marco prepares an entire spread of food for Baby.
- Academics: In "Match Maker", it is shown that Marco has good math skills, getting good grades (including an A in psychology) and in "Quest Buy", using the metric system to find a charger for Star's wand at Quest Buy. However, he admits in "Naysaya" that he doesn't know how to do long division. In "Britta's Tacos", Marco reveals that he got his equivalent of a high school diploma and finished his high school education early. In "Jannanigans", he demonstrates his academic prowess further by piecing together ancient archaeological clues from the strange hieroglyphs depicting Glossaryck.
- Strategic mind: Marco has somewhat of a talent for coming up with strategies, as demonstrated in "St. Olga's Reform School for Wayward Princesses" when he plans out Pony Head's rescue and in "Toffee" when he leads a resistance movement against Ludo. In "Divide" and "Conquer", he plans out ways to slow down Meteora's advance on the castle.
- Weapon-wielding: As of "Running with Scissors", Marco has become proficient at wielding a one-handed sword. As shown in "Divide", he can also skillfully wield an axe, a spiked flail, and a two-handed broadsword. He also wielded the Royal Magic Wand in "Deep Dive" and "Cleaved", albeit briefly, and was even the last person to do so.
- Survival skills: Sixteen years of experience hunting Hekapoo has turned Marco into a hardened survivor and warrior. "Divide" shows that he has retained these skills, even being able to fight on par with Star.
- Magic: Marco very briefly makes use of magic in "Deep Dive" and "Cleaved". In the former episode, he uses the magic wand to perform the All-Seeing Eye spell to find a missing Star. While using the spell, he temporarily gains crescent moon-shaped cheek marks. In the latter episode, he uses several of Star's spells and makes up some of his own while fighting Tom. Also in "Cleaved", he uses magic with Star (without the aid of the wand) to fuse the dimensions of Earth and Mewni together.
Names in other languages
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- Marco has appeared in every episode except "Is Mystery", "The Hard Way", "Moon the Undaunted", "Book Be Gone", "Puddle Defender", "Club Snubbed", "Stranger Danger", "Demoncism", "Starfari", "Ponymonium", "The Bogbeast of Boggabah", "Total Eclipsa the Moon", "Butterfly Trap", "Skooled!", "Bam Ui Pati!", "Tough Love", "Lake House Fever", "Down by the River", "Princess Quasar Caterpillar and the Magic Bell", "Ghost of Butterfly Castle", "Meteora's Lesson", "Junkin' Janna", "A Spell with No Name", and "The Monster and the Queen". His appearance in "Ludo in the Wild" was recycled footage from "Storm the Castle". He appeared in "Ludo, Where Art Thou?" only by proxy as a doll created by Ludo.
- Marco has the same birthday as his voice actor.
- In "Match Maker", Marco says that he has a dozen red hoodies, due to liking both the color red and hoodies.
- Marco's favorite color has varied depending on the episode/situation. In "Sleepover" and "Match Maker", he says his favorite color is red. However, in "Gift of the Card", he says his favorite color is plum at first, and later says it's Prussian blue.
- At the start of the series, Marco has a green belt with a white stripe in the Tang Soo Do discipline. As of "Red Belt", he is promoted to red belt.
- A green belt represents "the growth of a seed that sprouts upward, becoming a plant. Like the sprout, the student is becoming something greater." A green belt is roughly middle level.
- A red belt represents "a close, hot sun. This proximity to the sun represents the student's detailed knowledge." In standard forms of martial arts, a red belt is just below black belt.
- Marco's middle name is Ubaldo, as revealed on a test paper in "Match Maker".
- Marco's initials spell "MUD".
- Though he is Hispanic, the names "Marco" and "Ubaldo" are of Italian origin; Marco refers to Mars, the Roman god of war, while Ubaldo means “brave”.
- His middle name is said aloud for the first time in "Lint Catcher".
- At the start of the series, Marco is 14 years old. As of "Lava Lake Beach", he is 15.
- Marco once kissed a ninja at Karate-Kon 2012, as shown on the shirt he wears in "Cheer Up, Star".
- In the original pilot "Star and the Forces of Evil", Marco's pants are blue instead of dark gray.
- According to Marco's voice actor, Adam McArthur, Marco's favorite food is nachos, which was first demonstrated in "Party With a Pony".
- This has changed thanks to Janna; at some point before "Curse of the Blood Moon", Janna hypnotised Marco to feel sick at even the thought of nachos.
- Originally, in Daron Nefcy's version of Star without magical powers, Marco was named Sol, had an obsession with Dragon Ball Z (where his karate interest came from), and was more like an enemy to Star.
- As seen in "Cheer Up, Star", Marco's cellphone number is 555-0111, and his ringtone is "Space Unicorn" (which he claims is supposed to be ironic).
- However, when Star calls Marco's cellphone in "Storm the Castle" from the Diaz Household landline, she dials 573-1829.
- He suffers from motion sickness, as evident in "Brittney's Party", but in "The Other Exchange Student", he was fine riding to Isolation Point in his family's minivan.
- As seen in "Freeze Day", Marco has only greeted Jackie Lynn Thomas with a nod since kindergarten. At the end of the episode, he moves up to saying hello.
- In "St. Olga's Reform School for Wayward Princesses", Marco claims to have seen "every prison escape movie made".
- As shown in "My New Wand!", Marco can ride a unicycle.
- According to "Gift of the Card" and "Sophomore Slump", Marco's social security number starts with "904" and ends with "271".
- In "Naysaya", Marco is forced to reveal his deepest secrets and insecurities:
- Ever since watching a shark movie, he is afraid of the bathtub and only takes showers.
- He wears ballet shoes around the house (first revealed in "Gift of the Card").
- Until he started dating Jackie, he had never held hands with a girl, so he practiced with Ferguson.
- He always sits when he uses the bathroom to be polite to the girls living in his house.
- He cries when it rains.
- He still has his "wubbie", and he calls it Esteban (also revealed in "Crystal Clear").
- He used to think the "state of mind" was between Nebraska and Kansas.
- He has no idea how to do long division, despite being a straight-A student.
- He filled a notebook with his first name and Jackie's last name because he couldn't decide which looked better – "Marco Diaz Lynn Thomas" or "Marco Lynn Thomas Diaz".
- He kept Jackie's painting of a watermelon (which he originally thought was a moon) that she threw out in art class because it reminds him of his biggest fear: "that we're all alone on this tiny blue marble, floating in the infinite expanse of the universe, ripped from the center and ejected out into a black void for no reason at all. Players on the stage of the absurd."
- In "Heinous", Marco reveals he receives a monthly royalty check for $650 for "Princess Marco" merchandise based on his likeness. This becomes a recurring gag in earlier and later episodes:
- Prior to that episode, in "Goblin Dogs", he pays $650 for Goblin Dog merchandise.
- In "Pizza Thing", he pays a crane operator $650 to drive him and Pony Head to Emilio's Pizza.
- In "The Bounce Lounge", he provides the last $650 needed to pay off the Bounce Lounge's mortgage.
- In "Just Friends", he gets a medical bill for $650.
- In "Toffee", he uses the $650 to represent the Resistance members on a crudely drawn map for a plan to ruin Ludo's credibility as a ruler.
- In "Sophomore Slump", he uses the $650 to buy everything on a restaurant menu while on a date with Jackie.
- In "Marco Jr.", he pays Cobalt Ferrero $650 for a portrait of himself.
- In "Booth Buddies", he pays $650 to take pictures with Star in the photo booth.
- In "Butterfly Follies", he buys two pies at the Pie Carnyvale for $650 each.
- In "Yada Yada Berries", he bribes Apothecary Sherry with $650 for information.
- It is revealed in "Monster Bash" that Marco's karate stance has become a popular dance craze among Mewman and monster teenagers called the "Sword Hand Dance".
- Marco has a cat allergy, as shown in issue #4 of the Deep Trouble comic series.
- Marco's favorite karate move is the "block and claw", and his favorite spell is Rainbow Fist Punch.
- In the "FanCo13" live chat:
- Marco admits he's felt strange around Star ever since the events of "Blood Moon Ball".
- He also admits that he's not sure if he and Tom have become friends.
- He gets upset to learn that someone nicknamed him "Mango" (namely Oskar Greason in "Starcrushed").
- When asked about how he feels about Star, Marco admits that he still doesn't have an answer, saying that he barely had any time to process Star's confession (but also admits he's glad she told him).
- Additionally, in the "StarLIVE" live chat with Star Butterfly, although he has yet to give a finite answer on the subject, he casually remarks, "Who wouldn't have a crush on you?", hinting how he may feel about her.
- In the second "Marco's Spooky Stories" live chat, Marco admits that Star is "kinda cute", but when asked if he thinks she's "super cute", he becomes flustered and moves on to the next question.
- ↑ Physically, Marco is 15 years old as of "Lava Lake Beach". However, in Hekapoo's dimension, with the years he spent there in "Running with Scissors", he is in his 30s, as stated in "Marco Jr." and "Divide". He ages for an unspecified amount of time in "Ransomgram".