Kei Kurono is a high school student who becomes a participant in the lethal game orchestrated by the mysterious black sphere known as Gantz. His life comes to a sudden end when he and his childhood acquaintance, Masaru Kato, are struck and killed by a subway train while attempting to save the life of a drunkard who had fallen onto the tracks. Instead of ceasing to exist, Kurono awakens in an unfamiliar Tokyo apartment alongside others who have also just died, where they are forced into a cycle of hunting and killing aliens hiding within human society.

At the beginning of the story, Kurono is depicted as a selfish, cynical, and often crude individual. He is preoccupied with his own survival and driven by base desires, such as his sexual interest in a fellow participant named Kei Kishimoto, and he frequently acts as a reluctant hero who only looks out for himself. He lives alone in an apartment and is not on good terms with his family, contributing to his isolated and somewhat bitter worldview. His primary motivation early on is simply to survive the brutal missions so he can return to his ordinary life, which he views as an opportunity to lose his virginity and gain admiration.

As the series progresses, Kurono undergoes a significant transformation. After a disastrous mission where nearly all of his teammates, including Kato, are killed, Kurono becomes the sole survivor. This traumatic event fundamentally changes him. He grows more cautious and begins to prioritize protecting others over reckless displays of power. His relationship with a gentle, ordinary girl named Tae Kojima becomes a pivotal turning point in his development. For the first time, he experiences genuine love and a desire to live for someone else, which gives him a more optimistic and selfless outlook. He evolves from a self-absorbed teenager into a capable and respected leader of the Tokyo Gantz team, adopting Kato's previous philosophy that everyone should survive the missions together. His motivations shift from simple survival to protecting his loved ones and leading his team to victory with minimal casualties.

In his role within the Gantz narrative, Kurono is the central protagonist and often the key figure in the team's strategies and battles. He becomes a magnetic hero who inspires loyalty and courage in his teammates through his actions and determination. His key relationships are fundamental to his character arc. His bond with Masaru Kato is a core part of his story; Kato serves as a moral compass and heroic contrast to Kurono's initial selfishness, and the two share a deep, competitive, and brotherly connection. His relationship with Tae Kojima is a redemptive romance that humanizes him and becomes his primary reason for fighting. His dynamic with Reika, a celebrity who develops feelings for him, adds another layer of complexity, particularly when a copy of Kurono is created to be with her.

Kurono’s development is one of the most celebrated aspects of the series. He begins as an unlikable "jerk" and a "loser," but through the crucible of Gantz's deadly games and the loss of those close to him, he transforms into a heroic and admirable figure. He takes a significant level in badass, moving from a terrified novice to a masterful and inventive combatant. He also takes a level in kindness, abandoning his perverse and self-centered ways to risk his life for the sake of others. This evolution is portrayed as a realistic and gradual change, making his final status as a hero feel earned.

Despite being a normal human, Kurono develops into a formidable and resourceful fighter. He does not possess any superhuman abilities outside of the equipment provided by Gantz, but his greatest asset is his quick thinking and ability to improvise under pressure. He shows exceptional skill in using all of Gantz's weaponry, from the standard X-gun to the powerful Y-gun and Z-gun, and he becomes highly proficient with the Gantz sword. He is a master of unconventional tactics, often the first to figure out an alien's weakness or to devise a strategy that turns the tide of battle. Even without his Gantz suit, he is a capable and determined fighter, earning a reputation among his enemies as the strongest fighter on Earth. His will to survive and protect others is his most potent weapon, allowing him to overcome enemies that far outclass him in raw power.