As you can tell I'm new to the forums, although I've already made a couple of posts. However I've read Gantz from the start and am current on the manga. One thing I've noticed while searching the forums is the air of ambiguity surrounding Izumi's character. While reading, I was thinking about the introduction of Izumi, and the development of his mentality as the story unfolds. Now as far as I can tell, and we all have read/ seen, Izumi was once stuck in Gantz but had managed to achieve 100 points. Now when he did that I believe he chose freedom. Why? Because if you look at his character and his interaction with Kurono you begin to notice a couple of things, like when Izumi first brought up Gantz, the bomb was giving off a warning inside Kurono's head. And what seems clear by now is that those bombs give off a warning when someone who has no idea about Gantz is about to be told what it is by someone who is currently experiencing 'Gantz.' And remember, when Izumi met Kurono he only inquired about Gantz through the loose association of the fight he had gone through. Also, Izumi, in particular when he has that interciew, states that his excellent marks/ grades, prowess with sports, and even his physical beauty are all irrelevant, and that for a matter of implication, he feels empty inside like something's missing. And so, the further he digs into Kurono's life( and surfs the net) the closer he comes to finding out about Gantz, then eventually does-through a slight intervention of Gantz itself. Then as he experiences Gantz he realizes that's what he truly longed for all along.
And as for the reason he has so much contempt for Kurono, I think its because for once, there's somebody who's better at something than him, and he resents that. Even his small fascination with Tae stems from the fact that she likes someone other than him, and in turn coincidently the one she likes is Kurono. So I think when you look at it that way, its easy to understand the kind of character Izumi is, and why is development seems so static now that he's so active in Gantz.
Also I apologize if this seems like an absurdly lengthy post. :P