About Nishi in the recent chapters *Spoilers..
Oku brought Nishi back as a mcguffin of sorts a plot carrier. Kei had no real reason to revive nishi over tomb raider chick or kishimoto. Nishi wouldn't help the team atmosphere and this should have been apparent considering Kei already knew he was an opportunist.
Getting to my point, Nishi's been in hiding the whole phase II mission, and it seems pretty logical that he's going to take out the 100 pt. alien and either revive Kurono/Izumi or get the 100 pt weapon for himself. He really has no other reason for being brought back to GantZ unless he enlightens everyone about the various "Gantz teams" across the globe. It is apparent that he knows this information due to his discussion with Izumi aboard the train right before Izumi gets attacked by the vampires.