Ayumu Kato is a young boy and the younger brother of Masaru Kato, one of the central protagonists. He does not participate in the missions orchestrated by the black sphere, but he serves as a crucial emotional anchor for his older brother and a representation of the innocent lives that the main characters fight to protect.

Ayumu and Masaru are orphans who, prior to the events of the story, lived with a cruel and uncharitable aunt who would abuse Ayumu when he did something wrong. This harsh living situation was the primary motivation for Masaru to rent a small apartment where the two brothers could live together, escaping their relatives' mistreatment. Despite his young age, Ayumu demonstrates a maturity beyond his years, often telling his brother that he will find a job himself to help with the rent, showing his caring nature and his desire not to be a burden.

Throughout the narrative, Ayumu's primary role is that of a motivating force for Masaru. The older Kato's main resolve for coming back alive from the deadly Gantz missions is to see his younger brother again and ensure he is safe. Their relationship is one of deep mutual care, with Masaru acting as a guardian and protector, while Ayumu provides the emotional reason for Masaru to persevere through impossible odds.

Ayumu's experiences are marked by the repeated disappearances of his brother. During the Buddha mission, Ayumu woke up to find Masaru gone and went out searching for him. When Masaru was killed in that mission, Ayumu was left alone, presumably having to return to live with his aunt. A deeply emotional reunion occurs when Masaru is later revived by another Gantz participant; Ayumu sees his older brother for the first time in months, and they are able to resume their life together. This pattern repeats during the Nurarihyon mission, where Ayumu again awakens to find his brother missing and wanders the streets looking for him, even praying for his safe return.

Ayumu's development is subtle, as he remains a child throughout the story, but his resilience is notable. He endures the persecution and instability of his living situation while steadfastly believing that his brother will return to him. In the later stages of the story, during the Katastrophe, Masaru is seen protecting Ayumu alongside other children, highlighting that the brother's primary concern remains ensuring the boy's safety in the face of world-ending threats. Ayumu has no notable combat abilities, as he is never part of the Gantz program. His significance is purely narrative, serving as the personification of home, family, and the normal life that Masaru Kato strives to return to after each brutal mission.