Hmh, that's a good question. In the very end, you're the one who decides what kind of personality he has, but... well, one who's just completely ferocious and wants nothing but to fight can be a very dull character.
I like him as he is right now, though his temper is a -tad- too short. I do like that he's easily angered, and when he gets pissed off, he just goes completely nuts... and an addition could be that when he does so he goes into a kind of 'berserker rage' and will just keep going, no matter if it's friend or foe, though he'd have to get really pissed off for that to happen.
I think that, as soon as there's no battles to be had, he should be relatively calm and easy to talk to. I can't remember what kind of backstory he has, but I could kind of imagine him having been a very gentle and soft being, and having been forced by Deoxys to kill... well, everything, has gotten him angry at him for forcing him to go against his nature.
And the fact that he's angry at Deoxys for making him angry just makes it keep building. And that he's a Stage Two infected, has made him forget -why- he's angry, and only left the fact that he IS angry.
... Yeah, long rant, but that's how I see it XD