Well ive just red up to 230 and here are some interesting points:
When sakata arrives in gantz room for first time he says to cherry somthing along the lines of "this is familiar" "or i ve seen somthing like this before somewhere".
Also i was thinking that if nishi knows izumi then he probably knows about the lock on targeting with X-gun. It seems to me he had a really good strategy with the X-gun and the invisibility and using the other players as a distraction. Perhaps he got really skrewed in that mission because tanaka alien - could see him when invisible and could not be locked onto with x gun? Also his strategy is to let other players distract the alien but he is not necessarily willing to let them all die he did after all intervene in the fight with tanaka alien before anyone got killed! Also a lot of people say nishi is weak but perhaps he just got over ambitious and he wouldn't be the only one: kurono after all got too cocky in the buddha mission - lucky to pull off the giant buddha killl and then foolishly rushing in on the thousand arms boss and getting totally owned. So if you ask me nishi is pretty good at the game.
Also I think Izumi is the is the writers way of showing us that some one like kurono could have grown in a very different way. Kurono's attitude to the game in the early parts of the buddha mission was very much the same as izumi's general attitude. Perhaps if things had gone differently in that mission kurono would have become more and more bitter and narcisistic.
Those are my thoughts for now peace out