Hmm, true, I had forgotten that we got a chance to see Marten get angry on a couple cases. But even then, what'd we see? A few snarky comments and an urge to go drinking with his friend, which was quickly resolved into "Yes, maybe I could have handled that better."
Dora spent several hundred comics being insecure about her new relationship with Marten. Hannelore's been openly insane since her first appearance, bless her nutty little heart. Faye's issues have been prevalent for nearly the entire course of the comic. Steve got insecure and obsessive when he lost his lady-friend, Marigold broke out of her reclusive shell, hell, even Sven got to grow a personality and a conscience. Marten's worst freakouts last no more than a half-dozen strips before he's back to his normal, understanding self... he just copes with things inhumanly well. Seriously, there were jokes about Meena's roommate Dave being a serial killer because he's too nice, but really, Marten's only a half-step behind.
Honestly? I'd actually *like* to see Marten get his boundaries pushed a little more comprehensively. Not necessarily with Faye- or Dora-related relationship drama, but something that puts a crack in that shell of his. God knows that with all the crazy shit going on in his life, he's got enough reason to finally snap, and I'd be very curious to find out what that looks like.