(SPOILER)im pretty sure that only vexen dies...zexion, larxene and lexaes merely disappear to sum where IMO, axel is clearly seen in the kh2 trailer and the big explosion is probably not caused by marlucia but iin fact by that giant dusk he was riding...vexen was murdered by axel...so ya, im pretty sure that the surviving unknowns that appeared in COM are those five, and probably more will appear in the next game, since there are 13 members...