Who knows who the Heart is? (Well, Hoshino does, but she ain't telling).

Could be a new character. Could be someone we've never seen before.
Personally, I'm leaning more towards Komui, but that's just my own theory.

I've seen that Allen/Lenalee theory before, and I wouldn't mind it, except that it appears to have no real basis, especially since Apo-guy come along and shown us that they're innocence aren't the only sentient ones.