Ohlord, this is going to be a tough one~ x.x But I'd most defiantly say one of my favourite characters, "Vassily"; a character that plays an important role in a specific plotline me and my friend roleplay alone x3.
Sure, he's fun to derp around with and yell at Hawkeh (A friend of mine) on, along with drag into several roleplays
But oh gog, I just love his at-first seemingly friendly, kind, and innocent personality, but then as times goes on and he deems you to get in the way, he'll kill you off without a moments thought; almost as if the world was a game and the people in it were his pawns, while he is the clever chess-player, moving the pieces carefully into positions that much the opponent seem not to be able to stand a chance-
Oh, sorry, did I start blabbering here a little? xDD.