Kayo Sugimoto is an elderly woman who becomes a reluctant participant in the lethal missions orchestrated by the mysterious Gantz sphere. She appears to be in her seventies, with a small and average build, short white curled hair, and large glasses. Her typical clothing consists of a thick sweater and a long skirt, reflecting her age and ordinary life before being pulled into the game.

Before her death, Kayo was a completely ordinary grandmother. She died alongside her grandson in a major traffic accident while driving him home after a pleasant day together. Upon awakening in the mysterious room, she and her grandson find themselves forced to participate in a deadly hunt against aliens, as the group is assigned to the Tanaka Alien Mission, a violent and disorienting ordeal for two civilians so unprepared for violence.

In terms of personality, Kayo embodies the archetype of a sweet and caring grandmother. Her dominant trait is a deep, protective love for her grandson, whom she constantly seeks to comfort and shield from the horror surrounding them. She is fundamentally kind and motherly, but is also entirely out of her depth in the life-or-death scenarios presented by Gantz. Consequently, she is eager to go home and avoid the fighting, showing no desire to take charge or engage with the mission's objectives. Instead, she is quick to rely on stronger or more decisive individuals, deferring to the leadership of Masaru Kato, who convinces her and her grandson to wear protective Gantz suits. When not looking after her family, her primary motivation is simple survival and a wish to escape the nightmare.

Kayo's role in the story is a minor but poignant one, serving to highlight the brutal and indiscriminate nature of the Gantz game. She has no background in combat and her advanced age makes her physically infirm, limiting any ability to fight. Her reaction to danger is to flee or hide, showing no combat skill or initiative. In the mission, she and her grandson eventually break away from Kato's group. When the Tanaka aliens locate them, Kayo makes a final, desperate attempt to save her grandson by ordering him to run away, even as she knows she cannot escape. She is caught and seemingly mortally wounded, but in her final moments, she clings to her grandson, trying to shield him with her own body as the aliens kill them both.

The central and most defining relationship for Kayo is with her grandson, Ryota Sugimoto. She is his overprotective and doting grandmother, and their bond is the primary lens through which she is viewed. Her interactions with other characters are limited to following Masaru Kato's commands and displaying general kindness to others, but her world collapses to just her grandson once danger strikes. Kayo shows no notable abilities within the context of the Gantz missions. Her physical weakness and lack of combat training render her mostly helpless. Her only strengths are emotional and maternal, offering comfort rather than any tactical or offensive contribution to the team. Her development is minimal due to her short tenure in the game, though her final act of sacrificing herself in a futile attempt to save her grandson provides a tragic arc from a confused, scared civilian to a protective martyr.