Shion Izumi, also known simply as Izumi, is a central figure in the Gantz saga, recognized as an exceptionally skilled yet deeply troubled hunter. His background reveals that he participated in the Gantz missions prior to the main events of the story. During this initial period, he proved to be so competent that he was able to accumulate one hundred points. Instead of using those points to resurrect someone or obtain a powerful new weapon, Izumi chose the option to have his memories of the Gantz room erased and return to his normal life.

Despite being freed from the game, Izumi found his everyday existence to be profoundly empty and boring. He was a naturally gifted individual, excelling in academics and athletics such as soccer and basketball to such a degree that he faced no real competition, leading to an intense, all-consuming boredom. He eventually came across a website created by another former Gantz player, Nishi, which rekindled faint, forgotten feelings of excitement. This drove him to transfer to the school of Kei Kurono, a current Gantz participant, in order to get closer to the world he subconsciously craved.

On the surface, Izumi presents a flawless facade; he is polite, gentle, and kind, easily becoming popular and trusted by his classmates, even dating a girl named Ryoko. He can discuss sports and play video games like a normal teenager, leading people to see him as an upstanding young man who claims to hate war and violence. However, this persona is a complete fabrication. His true personality is cold, callous, and apathetic. He lacks empathy for human life, holds no moral or ethical qualms about killing, and views others with indifference. His defining motivation is a psychopathic need to return to the Gantz missions. He does not enjoy inflicting pain for its own sake, but rather craves the adrenaline and the feeling of being truly alive that only comes from fighting for his life on the brink of death. Violence and combat are merely the means to achieve that extraordinary thrill.

To force his way back into the Gantz room, Izumi devised a horrifying plan. After a failed attempt to provoke Kurono into killing him, he disguised himself and carried out a brutal massacre in the Shinjuku district. Using firearms, he killed or injured hundreds of innocent civilians, an act that resulted in the deaths of several future teammates who tried to stop him. He then kidnapped Kurono's girlfriend, Tae Kojima, forcing a confrontation that resulted in both his and Kurono's death, thus successfully returning them both to the Gantz room.

Once back in the game, Izumi's role is that of a powerful but volatile asset and a fierce rival to Kei Kurono. He is an excellent hunter, preferring to fight alone with ruthless efficiency and exceptional skill, often using the Gantz sword or other advanced weaponry. He is indifferent to the survival of his Tokyo team members, viewing them as weak and believing that he alone is strong enough to survive. His key relationship is with Kurono, driven by pride and a desperate need to prove himself superior. He becomes frustrated whenever Kurono outperforms him and relishes opportunities to compete directly against him, even when the mission objective becomes the killing of a human, Tae Kojima, which he ultimately does without hesitation.

Despite his psychopathic tendencies, Izumi shows small glimpses of complexity. He inexplicably saves and protects a small panda-like alien named Hoi Hoi, even playing with it affectionately. His most significant development occurs during his final confrontation with a group of vampires seeking revenge against the Gantz team. As he fights and kills several of them, his girlfriend Ryoko rushes to his side. In a sudden, uncharacteristic moment of selflessness, he shields her from a fatal attack, taking the blow himself. As he dies from his wounds, he comments that he is probably going to hell, an acknowledgment of his horrific actions that stops short of being full remorse, yet hints at some internal awareness of his own evil. In the end, Izumi is defined by his incredible combat prowess and his tragic, all-consuming search for a sense of purpose that he could only find through lethal combat.