Kuro and Krieg are the 3rd boss fight.
Haven't played much further than that. I did try to get most of everyone's attacks though. And this is pretty much how I like them:
Luffy: All around very good. Gear 2nd is crazy.
Zoro: Very powerful, but also very slow.
Sanji: For some reason the best character in the game. And that's not counting Diable Jambe.
Franky: Most powerful character in the game, but it doesn't matter, because he's so slow he's dead before he can get a hit in.
Brooke: Fun to play with. Pretty shitty though.
Robin: Very stylish attacks and one particular powerful combo, but that's pretty much it. She's dead pretty quick because she doesn't have much life.
Nami: Eh. She's perfect to quickly take out large amounts of fodder, but she's useless against bosses.
Chopper: Very crappy. He has one good attack and the rest is crap.
Usopp: Yeah, he sucks, but I heard he becomes much better once you unlock Sogeking, which I haven't yet.