Xenna Kuro [PLEASE CRITIQUE BIO]

Critique:

I can see already that you're wanting to get rid of the nekomimi aspect of the character, and I think that's a really good idea. What you might like to try out if you want to keep a cat-like feel to her, however, is to perhaps keep the cat theme, but forgo making it an integral part of her species. For instance, you can design her outfit to look more feline in appearance; you could include a belt that dangles behind her like a tail, you could have a tight, striped outfit somewhat reminiscent of a tiger, or perhaps something with fur laced around the neck, wrists or ankles. Another thing you might like to try out is, since she has sound-manipulation abilities, give her a set of custom headphones with a design that looks somewhat like cat ears.
Additionally, in wanting her powers to be sound-based, you could incorporate dance into her character, making her motions fluid and dance-like, but also delicate and calculated like those of a cat. 

I not being able to decide if she should be a Nobody of a human with powers, I instead offer up the alternative that perhaps she's a saved heartless. We've seen in the game that Sora could be saved from a heartless state, so perhaps craft a story scenario where your character, and perhaps some other humans, were turned into and then saved from a heartless state. Her powers could be granted as a result of the heartless form she took. 

Design-wise, her height and weight are fine (the weight's very accurate for someone her size and that's something a lot of people tend to underplay so good on you there). She could probably use a little more OOMPH to her clothing designs. I know you specify that she likes simple clothing, but the over-simplicity of it contrasts with the other KH character designs. Maybe try tweaking it so that you use patterns in place of heavy aesthetic details (like the zippers or fold present in canon KH designs). You could use a combination of simple, one-colour fabrics, and furs, a belt or simple, yet interesting, fabric patterns to keep her preferred design simplicity but make her fit in a little more with the cast. Based on the fact that she's meant to originate from Hollow Bastion, you could also give her a more elegant and perhaps steampunky or vine design to better indicate her world of origin. 

The multiple personalities thing is a lllliiiiiiittttllleee over the top, BUT it can be done in a variety of ways that work with the character.
For example, if you go ahead with the "rescued heartless" idea, the personality disorder could be a result of that--perhaps with other "rescued heartless" individuals having similar disorders as a result of the ordeal. You could also craft a story where someone was artificially turning people into Heartless in order to explain why these people were affected by the change and remain affected by it, but Sora wasn't. 
You could make the multiple personality disorder a result of her abilities, perhaps having them be (at first) too much for her to handle, so her mind fractured and she created an alter ego who was stronger than her.
No matter what you do, however, you MUST craft the alter persona as if it were an entirely separate person. Sadly that's something that this bio is lacking and it definitely takes away from the severity of the disorder as well as its impact on her life. 
I think you might also lack in an understanding of the implications of "multiple personalities" as your description of her personality states that she "changes personalities" but then only expands on personality traits that can become heightened (likely based on a situation). I would suggest first looking into and doing some heavy research on multiple personalities, because as you've written her now, she seems more like a multi-faceted character rather than one with multiple personalities. Mind you, a multi-faceted character is a very good thing and I would encourage you to keep her fluctuating personality as a part of her character, but I would also encourage you to craft an entirely separate character to put within her to act as the definite second personality. 
(As an example; someone who is traditionally submissive and perhaps not good at speaking up might fracture and create a second personality who is a more dominant and manipulative individual. The personalities would have different intellects and social skills, would likely have different sets of knowledge and one might be aware of the presence of the other without the vice versa.)

Personally I think it's a bad idea to have her be able to "control" Dusk nobodies. I think it might be a better idea to give her sound-manipulation powers an extension wherein she can act as something of a Pied Piper character. Like, hypnotic sounds. She could play music in a crowd of nobodies and have them lose control of themselves to the music. (Imagine her confronted by the main cast and her means of escape would be to crowd the nobodies around her, have them effectively start dancing uncontrollably, and then she could expertly slip through the dancing bodies while those pursuing her keep bumping into and having to fight the monsters.) This could definitely help with her passive attitude, wherein she doesn't fight people, she makes them dance until they're too tired to fight or she distracts them with dance so that she can escape and avoid a fight. 
Additionally, some extensions to her combat weaknesses are surely needed here. Apart from her lack of experience, I imagine she also wouldn't be able to hold her own against a primarily strong and close-fighting individual. And if this isn't the case, then why? Try to think of any combat scenario she might be confronted with and address exactly how she would get herself out of it, and if she couldn't, then why she couldn't. 

Backstory-wise, there are a lot of things you could work on. Primarily, I think the biggest problem happens to be how intricately her backstory ties into the main cast and their own stories and I think this is something you need to avoid. I also don't.... necessarily have a problem with people wanting to create characters t ship with their favorite canon characters, but there are very few instances where this is done effectively and I feel it's best to focus on the character you're creating first and foremost, rather than who you want to ship her with.
What's interesting about Kingdom Hearts is that all of the characters presented clearly have their own origins, histories and worlds that they both come from and (for some) have to get back to. Tying your character so deeply into the previously established canon character stories robs her of her own impact as her own character. Try to create a story for her that doesn't thrust her into the story so early on. Don't have her means of actions revolve around her crush on a canon character (unless she has "hopeless romantic" as a major personality trait, it's very off). Above all else, give her a goal. If you give her a goal, something that she alone wants to achieve, and something that indirectly brings her into contact with the canon cast, her own story will seem more real and she will seem like a more intricate character in her own right. She could team up with characters in KH to meet a certain end and then, when the task is done, she'll still have more to do, giving her the air of a real person with motivations, inclinations, goals, dreams and wants. 
It's best to come up with a character as if they're the main character of their own story, rather than something implemented into someone else's.

That's all the critique I can offer up at the moment. Hope it was helpful. 

(I would kinda be interested in coming up with a redesign for this character, but I'm sadly super strapped for time at the moment. If you're interested, however, maybe remind me again around mid-september and I can try my hand at coming up with a design for her?)