Kei Kurono is introduced as a fairly ordinary and unremarkable high school student who is cynical, self-centered, and preoccupied with his own desires, particularly those of a sexual nature. He lives alone in an apartment complex, having grown distant from his family, who he feels favored his younger brother over him. This lack of affection and validation contributes to his initial personality, making him a reluctant participant in any situation that does not directly benefit him. His life takes a dramatic turn when he encounters his childhood friend Masaru Kato at a subway station. When Kato rushes to help a disoriented man who has fallen onto the tracks, Kurono is reluctantly forced to assist. This act of heroism ends with both boys being hit by the oncoming train and killed.

Instead of dying, Kurono and Kato are transported to a mysterious room in a Tokyo apartment, where a large black sphere known as Gantz grants them a second chance at life. However, this second life comes with a brutal condition: they must hunt down and kill various aliens hiding within human society, using advanced weapons and protective bodysuits provided by the sphere. At the outset, Kurono is a selfish and often cowardly fighter, only looking out for his own survival and frequently complaining about the dangerous missions. He shows a particular interest in another new recruit, a young woman also named Kei Kishimoto, and often acts inappropriately towards her, hoping to start a sexual relationship. His motivations are simple: survive the mission, return to his normal life, and satisfy his personal urges.

The trajectory of Kurono's character is defined by a brutal but transformative journey. The first major turning point is the death of Masaru Kato during their third mission. Losing his morally grounded and heroic friend profoundly shakes Kurono, leaving him as the sole survivor of that operation. This traumatic event begins to chip away at his selfish exterior. In the following mission, he is sent alone as a form of punishment and, after a violent confrontation with a group of small, winged aliens, he is victorious. However, one alien survives and takes revenge by slaughtering several of his schoolmates. It is during this dark period that he meets Tae Kojima, an ordinary, homely girl he saves from the vengeful alien. What starts as a relationship of convenience or even a penalty game slowly blossoms into genuine love. Tae’s unwavering affection and kindness give Kurono a purpose beyond himself, and his love for her becomes the primary force behind his evolution. He begins to value life, not just his own but that of others, and becomes determined to survive not for his own selfish reasons, but to return to her.

As he grows more confident and skilled, Kurono transforms into a capable and respected leader of the Tokyo Gantz Team. He abandons his reckless impulsiveness for strategic thinking, often employing unconventional tactics to overcome powerful foes. He starts to embody the heroic ideals that Kato once held, holding meetings at his apartment to practice with the Gantz equipment and plan for missions to minimize casualties. His innate abilities with the Gantz weaponry, particularly the X-gun and the Gantz Sword, as well as his proficiency with the strength-enhancing suit, make him the team's most formidable fighter. His remarkable growth and willingness to protect others earn him the admiration of his peers, turning him into a reluctant but effective hero. After a particularly difficult mission, he earns enough points to request the revival of his fallen friend Kato and another former teammate, demonstrating how far he has come from the boy who only cared about himself.

Key relationships are central to Kurono’s development. His bond with Masaru Kato is the most significant, acting as a moral compass that guides Kurono’s actions even after Kato’s death. Kato’s inherent goodness serves as a constant contrast to Kurono’s initial cynicism, and ultimately, Kurono grows to embody the very principles his friend always held. In contrast, his relationship with Kei Kishimoto represents his early, more selfish and immature self, filled with jealousy and lust. The defining relationship, however, is with Tae Kojima. It is through her that he finds redemption and a reason to live beyond the violence of Gantz. This love story grounds him and provides the emotional core of his development. His journey is not linear; after retiring from the Gantz games to live peacefully with Tae, he is hunted down and killed by a group of vampires. Later, he is revived by Kato but with his memories erased from the point he left the game. Despite this, his connection with Tae proves powerful enough to rekindle his feelings. He faces a final, world-ending catastrophe, during which he fights to save humanity and rescue his beloved Tae, ultimately cementing his status as Earth’s greatest fighter and a true hero.