i believe that the chasers are good to a sense. my theory is that they are searching for all the keyblades at any cost (that's why you have to fight him) and bringing them to that desert world fearing that they will cause destruction. Although, Xehanort (who i think the old guy is) is interfering and trying to get the keyblades for his own reasons.
i think that Xehanort is the old guy and that the blonde soldier is not Roxas but has something to do with him (he's still alive, i can tell by his eyes) and the girl is totally new. And of course, the main soldier who was fighting the supposed Xehanort is the one you fight in Kingdom Hearts II: Final Mix +.