He clearly states his reason in the manga, so there is no speculation. Nishi knows things, and they need that information to help them survive.Kurono asked Old balls to revive Katou. He also asked Kaze to revive Nishi but its only speculation as to why.
Information like, "hey, dumbasses! If you spend just a few seconds playing around with your radar, you'll notice you can turn invisible to the aliens, and thus not die constantly." Things like that.
He made a UV radiation thingamajig, and gave them all a very nasty sunburn.Kurono was warned by his brother, Akira that vampires were after him so he went out and tried to make some kinda... weapon to defend himself. Kurono created some kinda light thingamajig.
The most popular character will always return.Q: Will Kurono be back?
Did you not see her in her very revealing tight black Gantz suit? Come on! Who else would the guys follow around.Q: Why was Reika named the leader?
A: Well... Katou just came back, Nishi is a kid, Kaze is nothing but a warrior, Cherry seems to be confused a lot, Sakata is a "dumbass," Inaba sucks, and would you make a 6 year old or an old man lead you?