Hmm, alright. So Nia's in the human world (I presume), forced to battle one of her own. why is he there? What's his mission? To study poor Nia? Take her back to the other side? I assume it's not to finish her, or else this is going to be an exceedingly short fic. No, you've got something intricate in the works. Always a good thing. You're good at delivering unexpectedly well-woven plotlines, so I expect that this could be your next gem.
To be honest, though, I think the pacing could have used some work. Missing transitions were a large part of that. There were several places where we just jumped from one scene to another without warning; from Nia's comment to the end of class, from the school to her attack, and from there to her apartment. That's a simple thing to fix, though; even an asterisk to divide scenes would signal everything to the reader. It's just a little off-putting without them.
The other pacing concern I had was the rate at which new information was thrust forth. In a couple of pages we found out that Nia's failing out of school, can't ever be happy, hates everything, has no heart (physical or spiritual), doesn't bleed, isn't human (although she used to be), and has some connection to another world that still isn't entirely clear to me. It was simply an information overload, particularly since many of the revelations didn't have a lot of prior evidence to support them. (If Nia had mentioned her lack of a heart after the non-bleeding attack, for example, it would have made more sense.) Finding out that Nia didn't have a heart as she was walking away from class... I don't know. It all just seemed rather... rushed. Maybe I'll have a much different view of it once I take a second look at it. After all, I've gotten about three hours of sleep in the last 48, so I'm not in any condition to comprehend anything.
The good news is that things flowed excellently once the fight ensued. Your action sequences were easy to digest, to the point that there was no need to explain Nia's voice power. I could visualize the thoroughly cracked glass with ease, and even the vaguely referenced "seal" was simple enough to grasp. While Axel, to someone who hasn't played all the KH games, is a little unfamiliar, you've set things up so that readers new to the fandom don't have to know everything (or anything) about him yet. To make a long story short, action is one area in which I think you've improved dramatically, and that scene showed your talent very well.
I think that this fic has a lot of potential, really. The beginning kind of surprised me, but you certainly found your stride in the second half of the chapter. No complaints there. That strong conclusion makes me wonder what you've got up your sleeve. Forcing our main character into submission was a simple first move by Axel, but what does he have planned for Nia? For that matter, what secrets lie in Nia's dark past? Only time, and you, will tell. I look forward to it!