Jouichirou Nishi was a seasoned veteran of the Gantz world who had been there for over a year before the series began. He met his end while trying to escape from police officers after stealing something, by jumping off an apartment building roof. Despite being reluctant to share information about himself or the Gantz, Nishi runs a website where he posts details about it (while adhering to the rule that no outsiders should be informed, which could lead to fatal consequences). Interestingly, Gantz refers to him as "Mr. Nishi," even though he is known for his informal or derisive attitude towards most other participants.
Nishi is killed at the beginning of the Tanaka/Suzuki Alien mission, but after six months of existing only inside Gantzs memory banks, he is revived by Kaze at Kuronos request following the end of the Oni mission. Ostensibly, Kurono hopes that Nishi will be able to provide information about Gantz to the game players.
After being ostracized and bullied by his classmates, which ended with them hurling him out of the classroom window, he has killed all of them (except one girl who had a crush on him and did not support the rest in wanting to chase him out of the school). After this, civil armed forces are sent to kill Nishi.
Nishi manages to kill many of the men sent to kill him, but after an intense battle with the armed forces he is severely wounded and was only able to avoid death because Gantz called the Hunters for a new mission.