Kei Kurono is introduced as a seventeen-year-old tenth-grade student living alone in an apartment complex, largely estranged from his family, including his deceased younger brother, Akira. His life is irrevocably altered after he and his childhood friend, Masaru Kato, are killed by a subway train while attempting to save a disoriented man from the tracks. Instead of dying, Kurono awakens in an unfamiliar room with several other recently deceased people, where a mysterious black sphere known as Gantz forces them to hunt down and eliminate aliens hidden within human society.

Initially, Kurono is portrayed as a deeply flawed and selfish individual. He is cynical, preoccupied with sexual desires, and often acts cowardly, displaying a general indifference toward the suffering of others. He frequently complains about his circumstances and is reluctant to put himself in harm's way, only acting when forced to by the situation. This behavior stems from a history of neglect and a lack of affection from his parents, which has left him feeling isolated and resentful. His early interactions are marked by jealousy, particularly when a female teammate, Kei Kishimoto, shows more interest in the kind-hearted Kato than in him. Gantz itself acknowledges his initial unpleasant demeanor, referring to him with derogatory nicknames like "Bitch" and "Loser" before settling on his name.

Kurono’s primary motivation shifts dramatically over the course of the narrative. What begins as a simple drive for personal survival and a chance to prove himself by wielding Gantz’s powerful technology gradually evolves. A pivotal turning point occurs during his third mission, which results in the death of Kato and all other participants, leaving Kurono as the sole survivor. This traumatic event strips away his earlier arrogance and makes him fear the missions, forcing him to rely more on strategy and brains rather than reckless force. A more profound change takes hold when he meets and saves a girl named Tae Kojima. His genuine romantic relationship with Tae gives him a new, positive purpose and a powerful reason to survive, reshaping his worldview from one of pure self-interest to one focused on protecting others.

As the story progresses, Kurono grows into a capable and respected leader of the Tokyo Gantz Team. He adopts Kato’s former philosophy of ensuring everyone returns alive from missions, holding strategy meetings at his apartment and demonstrating a natural tactical acumen during fights. He becomes a hero to other members, using unconventional tactics and his innate physical prowess to overcome overwhelming odds. After successfully building a strong team and reviving several fallen friends, Kurono chooses to retire from the Gantz games, opting to have his memories of the experience erased so he can live peacefully with Tae. This peace is short-lived, as he is tracked down and killed by a group of vampires who are hostile to Gantz hunters.

His role in the story is that of a reluctant hero whose character arc forms the emotional core of the series. Key relationships are central to his development. His friendship with the morally upright and selfless Masaru Kato serves as a constant contrast to his own initial pettiness, and Kato's death is a major catalyst for his change. His loving relationship with the ordinary and homely Tae Kojima is the cornerstone of his redemption, providing him with genuine affection and a reason to become a better person. Later, he becomes an object of affection and admiration for another hunter, Reika, which creates a complex dynamic when a copy of Kurono is revived to be with her.

As a combatant, Kurono is notable for his adaptability, creativity, and resourcefulness rather than any superhuman innate power. He relies on the advanced weaponry provided by Gantz, including a powerful gun that can target enemies with precision and a specialized suit that greatly enhances his physical strength, speed, and durability, allowing him to survive falls and powerful attacks. Over time, he masters a variety of weapons, including an X-gun and a Gantz Sword, and becomes proficient in using the suit's abilities. Even when his memories are erased after his revival, his core survival instincts and quick thinking remain, allowing him to handle dangerous situations. By the end of the story, he is recognized as the strongest fighter on Earth, facing down a powerful alien champion in a final battle to save humanity.