Through no desire of his own, Kei's investigation of a run-down apartment building has led to a rather unfortunate discovery. Not only is it chock-full of the strange bird aliens that they've been sent to fight, but the boss bird is much bigger, tougher, and scarier than the regular ones, and those were quite bad enough. But when the chips are down Kei still manages to be one of the most resourceful characters in the series, and he finds a way to survive and win.
But despite all that he still finds that in the things that matter, in his real life, he's still nothing in the eyes of others when all he really wants is proof that he exists. In the room, in the world that Gantz has given him, he's powerful, important, and above all else - alive. Outside of that room, back in the real world, he merely exists, a person to either ignore or stomp on, nothing else. This dichotomy causes him to crave that reality, that powerful feeling of existing and being outside of the strictures of civilization. Kei's losing his humanity and becoming something else, perhaps even something monstrous, but he relishes the transformation where the others fear or deny it.