Of course I understand that's what the Joker is (in this film, at any rate). He doesn't need an arc and that's fine. What I'm getting at is that the character arcs of others around him in The Dark Knight are unsatisfying or unconvincing (especially Two-Face). I mention the Joker purely to point out he doesn't qualify for a character arc in the absence of a satisfying one for Batman. I probably should have made that more clear.