Ayumu Katou is a supporting character in Gantz, best known as the younger brother of the primary protagonist Masaru Katou. He is a young boy with black hair and black eyes, appearing to be of elementary school age. His role in the narrative is not that of a Gantz player; he never enters the mysterious black sphere's room nor participates in any of the deadly missions against aliens. Instead, his importance is entirely emotional, serving as the central motivation for his brother Masaru to survive the brutal Gantz games.

The background of Ayumu is marked by tragedy and hardship. He and his older brother Masaru lost their parents at a young age, which forced the siblings to live with various relatives. The most prominent of these living situations was with an aunt who was cruel and uncharitable, subjecting Ayumu to abuse whenever he made a mistake. This harsh environment ultimately led Masaru to take drastic action; he rented a small apartment for the two of them to live in on their own, escaping the mistreatment of their aunt. Despite his youth, Ayumu frequently told his brother that he would find a job to help pay the rent, demonstrating a mature sense of responsibility and a desire to lessen Masaru's burden.

Ayumu's personality is defined by a quiet resilience and an unwavering faith in his older brother. He endures persecution and the injustice of their living situation with a steadfast belief that Masaru will always return to him. This loyalty and dependence create the primary emotional anchor for Masaru, who fights desperately in the Gantz missions specifically for the chance to come back alive to his brother. The plot often shows Ayumu waking up to find Masaru inexplicably gone, as his brother has been teleported away to a mission. He is shown wandering outside, searching for Masaru and praying for his safe return, completely unaware of the violent alien conflicts his brother is engaged in.

His key relationship, naturally, is solely with Masaru Katou. This bond is the core of his character. The tragic turning point in their story occurs when Masaru is killed during the Buddha mission. For months, Ayumu is left alone, believing his brother has simply disappeared. His fate is reversed when Kei Kurono, Masaru's fellow Gantz player, uses his accumulated points to resurrect Masaru from the dead. The reunion between the two brothers is a significant emotional event, and they happily return to living their old life together in the apartment they had promised to share. However, the cycle of loss repeats when Masaru is once again taken from the apartment to fight in the Nurarihyon mission. Once more, Ayumu wakes up to an empty room and wanders the streets searching, clinging to the hope that his brother will be returned to him. Masaru survives this mission as well, and they are reunited again.

In terms of development, Ayumu himself does not change dramatically, as he is a static character whose primary function is to be a motivation for others. His growth is more about endurance than transformation; he continues to hope and wait for his brother despite repeated abandonment. During the catastrophic alien invasion known as the Katastrophe, his role evolves slightly as he is seen being protected by Masaru, alongside other children, showing that his safety remains Masaru's ultimate priority. In the final arc of the story, after Masaru returns alive from the alien mothership, the brothers are finally able to live together in peace in the apartment they had always dreamed of.

Ayumu possesses no notable abilities related to combat or the supernatural. His strengths are entirely emotional: his capacity for hope, his loyalty, and his role as the human cost of the Gantz games. He is a representation of the normal life and personal connections that the Gantz players are fighting to protect and return to.