I apologize if this turns out to be incorrect, off-topic, or if it is common knowledge.
According to "Gantz Labo", a Japanese fansite, Oku Hiroya based the character of Nishi off of Sakakibara Seito, a teenaged murderer. I know that there are many obvious examples of him basing his characters off of popular icons (Lara Croft, Golgo13), but this one seems to be lesser known.
Can anyone think of anymore "Gantz" pop-culture references like this? (Besides the aforementioned obvious ones, naturally.)
Wikipedia on Sakakibara.
Photograph of Sakakibara.
Gantz Labo - Japanese.