1.Xemnas: A great character. Xemnas is higly offensive and is perhaps one of the most useful characters. Pretty much useful for any situation, Xemnas is also the one of the strongest. He has High Attack Stats for attacking and High Magic Stats also. His other stats are average.
Tips: Equip weapons that have good aerial combos because I have noticed him better for aerial fights. Equip as much magic as possible. He is great for bosses, and he can also sweep large numbers of enemies.
2.Xigbar: He is incredibly fast, and his combos are wonderful. His range is the best among all characters. There is a downside. He has low attack power and low in everything else too. He is also the worst one to use when playing solo. He is very fun to use for multiplayer but I would steer away from him for solo playing.
Tips: Only use him for Multiplayer. If you use him for solo, your'e playing with fire. Almost any weapon is good. I reccomend that you stick with ground combos since he's not that good with aerial combos.
4.Vexen: Eww. Vexen. First of all, he doesn't ,makEmuch sense. How the hell can a guy with a shield have a defense of 4?! Steer away from he's slow, has low Attack. Pretty much the only thing he's good at is magic and your'e better off with Donald Zexion or Mickey for that. He can work in Multiplayer, though I still hate him.
Tips: Use him only in Multiplayer. I hate him. That is all.
Since I finished the game already (Japanese) I was able to use all of them. I just have to say that Vexen is the character I only tried once and hated immediately. I like most though.
Info in the spoiler:
Roxas, Xion, Axel, Xaldin and Sora (yes, Sora) for balance. Donald (yes, Donald), Zexion, Demyx and Mickey (yes, Mickey) for magic. Larxene, Xigbar, and Zexion for speed. Xemnas, Lexeaus, Saix, DW Roxas, Riku (yes, Riku), Goofy (yes Goofy) and Marluxia for strength. For combos, Marluxia (unbeatable ground combo), Zexion (book combos), Xemnas, Xigbar (gun combo), Mickey (highest jumper), and DW Roxas, Larxene and Demyx. I just hate Vexen. Goofy and Lexeaus reign in defense.