I know I am not the first to say this, but if its any consolation I came up with it on my own and only discovered it had been done before afterwards. Before season 2 of the Owl House comes out I would like to put my two cents into this theory. The main parties involved in this theory are Dave Lucitor (a caucaian, brown haired, blue eyed mewman, with round ears which because of his mewman heritage has potential for dormant magical powers), Wrathmelior Lucitor (a huge red skinned demon with three vertically slanted and slit yellow eyes with no pupils, dark pink hair, long pointed ears, and giant horns on her head, and demonic powers such as super strength and weather manipulation.{We know demonic powers are separate from magic because when all the magic left stars universe Globgor still had his size manipulation so demonic powers and magic are different}), Thomas Draconius "Tom" Lucitor (light purple skin, light pinkish-purple hair, three pinkish red eyes with more human characteristics, smaller horns then his mom, pointed teeth, and pointed ears although less demonic then his mom, and a lot of demonic powers such as pyrokinesis, telekinesis, levitation, Curse infliction, and Geokinesis), Janna Ordonia (a Filipino human with brown eyes, dark blue {most likely black dyed blue}, round ears, and no magical abilities), and finally Boscha ( A supposed witch with light pink skin, dark purplish pink hair, and three human like eyes that are a greyish blue, rounder teeth, and round but pointed ears.)

Before I continue I would like to clarify that in my theory Luz and gravity falls earth is a different earth than Marco’s earth, they exist in the same universe, but are two separate realities. So my theory is that at some point after Star and Marco and all the other dimensions collide Tom and Janna fall in love, I can see this happening very easily from the canon we have, and the combination of there genetics lead to Boscha (who so far has no canon last name) I can easily see her skin being a combination of Tom and Janna’s, her ears would be from her 1/4 demon status, her three ears would be from her father but are more human in appearance then Tom’s because of Janna, her hair is from her father and grandmother, and her blue eyes from her grandfather. Her magic could source from two stems or a combination of both, one it isn’t magic it is her demonic heritage from her father, or two just like Luz who has no magic she has studied runes and other magical objects and are using that, again t could also be both.
So why is she in the Demon realm and why is she so rude to non-witches when she isn’t one herself, my answer, she’s adopted. Some point after she is born, perhaps around two to three, she is somehow lost and somehow ends up through a dimensional rift or a portal and is found in the Boiling Isles 10 years prior to the series (as in time travel) by a high class female witch. Clearly not human and having near identical pointed ears to a witch the woman would assume Boscha is a witch, and would adopt her as her own, Boscha would only remembers her first name so that is what her mother would call her. Boscha would show her “magical” alibies early on making her mother very proud and would brag about it to the Blights. She would eventually notice that Boscha’s magic doesn’t quite work the same and would teach her runes and other things to compensate. She wouldn’t remember her family so she wouldn’t know her 3/4 human heritage and would learn her bigotry from her mother. Weather she ever learns the truth, we will have to wait and see. Her bio parents would probably search everywhere for there daughter but even if they could get to the demon realm and happened to bump into her they would be looking for a 3 year old, not 13 and might not recognize her.
Anyway thats my theory.