- “See, Marco? You're adorable when you laugh at yourself.”
- ―Jackie, to Marco[src]
Jackie Lynn Thomas is a student at Echo Creek Academy. Marco has a crush on her at the start of the series, of which she is made aware in "Sleepover", and the two have a romantic relationship from "Bon Bon the Birthday Clown" to "Sophomore Slump".
Jackie has wavy, shoulder-length platinum blond hair (which hides her ears) with an aqua streak on the left side, tan skin, mint green eyes, and freckles. She is of medium height and has a slim, curvy body. She wears a green and white raglan shirt, an orange seashell necklace, cyan jean shorts, green and white knee socks, and blue and white sneakers. Her eyes are typically half-closed, but, like Star, when they are completely opened they overlap her bangs. In "Freeze Day", Jackie appears as a little girl and preteen without the aqua streak in her hair. In Brittney's Party she wears a purple shirt, blue jeans, brown shoes, and a flower in her hair.
In "Just Friends," it is revealed that she has multiple piercings in her right ear and wears a shirt that Star made for the concert.
In "Sophomore Slump", Jackie wears a blue dress and red shoes.
Marco has had a huge crush on Jackie for years, but he never had the courage to talk to her or get her attention. However, Jackie acts friendly and chill around him, which is usually how she acts around everyone. In "Interdimensional Field Trip", she sits next to him on the bus ride home from a field trip. Marco eventually reveals his crush on her in "Sleepover", much to her surprise. When Marco admits that he wants to get to know her better, Jackie reacts by smiling at him, and later admitting to no longer knowing whether or not she had a crush on Marco or on a guy from skate camp (as she previously mentioned).
In "Bon Bon the Birthday Clown", she goes on a date with Marco and convinces him to let her teach him how to skateboard. The two also share their first kiss and officially become a couple. In "Just Friends", Jackie attends a Love Sentence concert with Star and Marco. When Marco feels like he's not fun enough to hang out with the two of them, Jackie assures him that she likes hanging out with him. She and Marco kiss twice at the concert. In "Starcrushed," Jackie tries to get Marco's mind off of his and Star's strained relationship after the events of "Face the Music", and she also acts more casual and dorky around him.
In "Sophomore Slump", Jackie realizes that Marco's adventures on Mewni have distanced him from his friends and family on Earth, and she breaks off their relationship before it causes them both misery in the long run. According to Marco following his and Star's return to Earth, another reason Jackie broke up with him was because of his growing feelings for Star.
In "Britta's Tacos", Marco and Jackie meet up again for the first time since their breakup. After agreeing they weren't right for each other, they decide to start over as good friends.
In "Cleaved", Jackie lends Marco her skateboard so that he can reach the dimensional rift and reunite with Star.
Jackie thinks of Star as really cool, like the other students at Echo Creek. Star thinks the same of Jackie, and the two hang out together in "Sleepover" and "Just Friends". In the latter episode, the two share a love for the Love Sentence. However, Star becomes jealous of her growing relationship with Marco, and she even uses a spell to spy on them when they go on their first date.
In "Starcrushed", when Star reveals to Marco that she has a crush on him, Jackie looks shocked by the revelation.
In "Sophomore Slump", Jackie alludes to the fact she knows Marco also likes Star, being one of the factors in her breaking up with him so he can be happier.
In "Britta's Tacos", Jackie and Marco reunite for the first time since their breakup, and while saying goodbye to Star, Jackie tells Marco that Star is "a special girl" and playfully tells him to not mess things up with her too.
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- In "Freeze Day", it is shown that Jackie has known Marco, Janna, and Justin since kindergarten.
- It is also shown that her name may be spelled without the "C", as seen briefly in the background. This could either be an animation error or an intentional misspelling due to Jackie's young age.
- Jackie's voice actress, Grey Griffin, also voices Moon Butterfly, Hope, and Andrea.
- In earlier episodes, the aqua streak in her hair would accidentally appear on the right side of her head instead of the left side as an animation error.
- In "Bon Bon the Birthday Clown", Jackie kisses Marco for the first time.
- In "Just Friends", Jackie reveals that she is a fan of Love Sentence and that her uncle works for a company that makes skateboards.
- Her favorite Love Sentence song is "Just Friends".
- Her name may be short for "Jacqueline", since this is what Naysaya refers to her as.
- As of "Britta's Tacos", Jackie is studying abroad in France, and she is in a relationship with a French girl named Chloe.
- According to Star and Marco's Guide to Mastering Every Dimension, the aqua streak in Jackie's hair is natural. However, the flashbacks in "Freeze Day" show that she did not have this aqua streak until adolescence.
- According to the same source, Jackie's spirit animal is a basketful of kittens.
- Jackie in "Just Friends": "My uncle works for the company."
- Charlotte Jackson (2019-04-29). oh hey, the star vs episode “britta’s tacos” aired! as a bi person myself, it was cool writing/boarding the scene with jackie and her girlfriend chloé. :^D big thanks to daron & the rest of the crew for making that happen!. Twitter. Retrieved on 2019 April 29.
- Season 2, episode 17: "Sleepover"
- "Season 2, episode 26: "Naysaya"
- Season 2, episode 27: "Bon Bon the Birthday Clown"
- Season 2, episode 39: "Just Friends"