I would say I'm decent with every character except Sieg, Lime, and Eir. I'm not too great with them.
Sieg because I can't do all those crazy complicated stuff and he sort of feels clunky to me. (but I do love AS Sp Sp-ing stuff. Sounds awesome.)
Lime, just because I don't like his play style.
Eir, I focus on attacking with her (limited) attack skills, and sometimes forget to heal.
Oh, and Tia I can't do crazy ridiculous combos with either, but I'm okay with her I guess.
Everyone else I'd say I'm knowledgeable in.