First off a semi-important Dark Knight YAY! (gimme 5 my inner tactician is currently laughing with manaical glee)
As for his character, so far I like it, cold calculating genius with a hefty dose of awesome points (just look at that armor and his dialogue so far). I really hope he fufills the Deadpan Snarker trope. The Japanese VA choice is completely appropriate from what we've seen of his character (his tome had better be called leathality or the death note for reasons). As other people have said he seems to have very little problem with threatening Kamui's continued existence and slinging around powerful (dark?) magic. Cold, calculating, pragmatic, what's not to like?
All in all 10/10 would use Book Bro