Nishi*Spoilers from 219*

Why would Kei bring back Nishi? i can think of a few reasons but none of them make sense.. anyone got any ideas ?

good job there, you managed to put a spoiler warning and the spoiler in your topic title. bravo.
Kei probably thinks that Nishi knows a bit more then he knows. since Nishi had that website up, and it eluded to things that he couldn't make too much sence of. you know the whole 'end of the world' thing.
Also probably feels sorry for the kid, seeing as how close he came to geting out, and the fact that he wasn't really as hard as he acted out. could be out of sympathy

i think that kei belives that nishi know more about gantz then kei does.......

Kei feels guilty that everyone there gave up their freedom in order to give him some revivals, so he's reviving the person he thinks will give them all the best chance to survive all of this. With Nishi's knowledge, he'll be able to master some of Gantz's intricacies and also solve some of the mysteries of what Gantz is. Besides, he hasn't gone yet, so no one knows who he'll revive next.