Is ESP The Powerful Weapon?
Sakata hadnt been inside gantz before but he said a guy taught him how to use ESP and made him kill the guys that were bullying him
and when he got into the gantz room he said the ball seemed familiar
and when you think about the whole sakatas master made him kill people (well he didnt strictly make him do it but sakata wanted the powers to kill them and his master let him have them) it seems like he mustve been a pretty fucked up dude, and any one who can not only get 100 points in gantz but opts to stay in with a powerful weapon they gotta be pretty fucked
and its pretty obv now that izumis sword is not the powerful weapon
just a thought... now i await the first bastard to prove me wrong and spoil my ideas :)