Sora is Light. Mickey is Darkness. Riku is Twilight (Dawn). And I would say Aqua is Twilight (Dusk) considering where she is at the end of BBS. or Terra...Maybe Xehanort. Considering if we are choosing heroes, then most likely Aqua since she has been in the Darkness for a long time. She is like the light left after the sun has set. Struggling, but still there to light the way.
Dusk is the point just after the sun sets and there is still light, but it is slowly getting darker. Dawn is just before the sun rises, where it is dark but slowly getting lighter. Both are Twilight. Riku strives for the Light, which is why he chose Dawn. Dusk is slowly moving towards darkness, which doesn't necessarily mean becoming evil.
Since the elements in this game seem to be split into three though, there is Light, Darkness, and Twilight. Then Twilight can be split into a lighter side (Dawn) or a darker side (Dusk)