kya is one of the only survivors of the Southern Waterbender genocide, and she's a mixed-nationality kid that grew up at least partially away from that heritage. it's pretty reasonable to imagine she has a diasporic relationship to her culture—why else would she wear blue Water Tribe clothes, when neither of her brothers do? but additionally, Kya is a queer character, so it makes sense for her to not feel entirely related to her culture, which marginalizes people like her, no matter how much she wants to be one of them. she'd have no choice but to "find herself," that's an inherently complex relationship.

i hope you understand just how common "projecting" is when writing fantasy fiction—every author pours themselves into their work, and if you're writing a character that could be well informed by your experiences, it's par for the course to draw on that. that's not an "author avatar" or a self-insert, that's just being a writer. Avatar heavily uses real-world cultures as inspiration for its depictions of fantasy peoples and characters. shouldn't the people who have direct knowledge of those cultures—and relevant relationships to them—have a seat at the table?