1. The concept of three/trinities in Kingdom Hearts can also extend to the three major Keyblade wielders:
Sora (Light) -- representative of belief and trust in authority; he does as he's told and never thinks to question otherwise what his elders (and, as far as he knows -- and never seeks to question -- betters) have told him about anything in the world. He's acceptance and loyalty, unwavering.
Riku (Dawn) -- somewhere between light-and-dark, but still an obligation. Riku doesn't do things because he chooses to -- he does them because he feels he has to. He also still trusts some authority (namely the King), but also waffles in that gray area of doing it himself. He is still dependant on what people tell him and his instructions, though to a lesser degree than Sora.
Mickey (Darkness) -- free choice. Free will. Which is kind of hilarious in so many ways because it is Mickey Mouse we're talking about, but. He even has a line that is essentially this, that there is the freedom to choose, and that's what's important. Mickey is completely independant, operating essentially on his own agenda: he answers to his friends, but he never feels obliated to take care of them when they can fend for themselves, OR even necessarily explain everything. Mickey is about letting people make their mistakes and make their choices from there. LIKE EVERY AUTHORITY FIGURE he does not always succeed at this -- but he alone, out of everyone, realizes consciously from the start that darkness is not always bad. It's just scary. He does not rely on anyone's advice or opinion to sway his choice; he takes what he knows and applies what he believes to what he does.
2. As much as Sora provoked Demyx into the last fight, Donald was just as bad, if not worse. This is yet another example of why Donald is a terrible authority figure, sob.
HOWEVER: there are a series of things that happens immediately surrounding Demyx's death that are interesting
1. Sora is outright aggressive/angry afterwards, demanding that any member of the Organization interested in taking him on comes forward -- however, he looks outright surprised with himself and sheepish when he's admonished not to antagonize them.
2. Goofy's "death" -- both Mickey and Donald react with passionate anger, both of them running off immediately after. Sora, on the other hand, is crushed; he gets up reluctantly and is obviously not comfortable with leaving Goofy behind, though he does in order to catch up with his "living" friends.
3. The confrontation with Saix, and the half-given information about Kingdom Hearts and the Organization's plan for it (which is to say: no wonder Sora thinks they were doing bad things the whole time -- he gets a lot of cryptic things about using Kingdom Hearts, and then immediately the game launches into the section where the endbosses for the revisted worlds are various members of the Organization).
4. Sora's first glimpse of Roxas, and automatically knowing his name, somehow. Timing is very interesting in this point; we see a much more sullen and angry Sora for a few seconds and a short time later are reintroduced properly to Roxas as a plot point/character.
Roxas, beng a Nobody, is technically of the darkness, and is, at this point, not completely reintegrated with Sora. (That won't happen until their confrontation in The World That Never Was.) However, his presence alone sort of ... not exactly darkens Sora, but does swing him a little more to dawn/twilight, rather than solely the light.
3. Fighting 1000 Heartless is a wonderfully cathartic experience. :( RELATEDLY, Sora is one powerful little bugger. He's a lot of essentially brute strength wrapped up in a young and impressionable package; he wields an object that is probably the closest to a truly holy item that his canon can have and fights with an instinctive ease. I am so very curious for more information about the Keyblade War and the other bits and pieces we're seeing in the tralers for the games coming out. SOB, I AM SO CURIOUS -- also, while I am not and will never be a fan of Atlantica, I am very curious about what Triton knows. His attitude is definitely one of experience when it comes to the Keyblade, and he's of the right age that he would have very likely been around for the War. (How old can merpeople get, anyway? In the original fairytale, it was something like 300 years, but Ariel's like. Sixteen. A-and then she gets married and oh wait huh.)
IN CONCLUSION: REPLAY IS STILL HAPPENING. I LOVE MY CANON.