I think, the main flaw with Arthur at the moment, is that he doesn't seem to a clear path or mindset. A few events here and there occur, but none of them really change his overall personality or views, and appear a bit random. Like when he ate the Tethusial, it was absolutly hilarious, but all it gave the impression of was that Arthur was either very sick or bonkers. I couldn't really gleaner what significance it had. And when he attacked Isobella for 'stealing his kill'. That made me laugh quite a bit, but you never really explained a reason. Once again, instead of getting a clue of Arthur's past or hidden truth, it made me think that he might be a bit of a jerk. Don't get me wrong, Arthur was awesome in Quest #17. But I can't see anything big building up with him. I'm not saying that he needs to turn out to be a Ex-Millionaire-Genius-Playboy-Philanthropist until he lost his memory or something cheesy like that. But if he is going to do some slightly weird things occasionly, maybe consider writing a sort of direction for him, or it just comes off as a bit of a joke.
EDIT: Argh, I'm sorry, that sounds really cutting doesn't it? I mean no offence at all, sorry if that was a bit rude.