Ayumu Kato is Masaru Kato's younger brother.
While he doesn't appear in the Gantz room nor participate in any of the missions, he is an important character as he is Kato's main resolve for coming back alive. Early in the series, Ayumu and Kato were staying with their cruel and uncharitable aunt who abused Ayumu when he did something wrong. Kato rents a room a little later for them to live in to escape their aunt. Ayumu kept telling Kato that he'll get a job himself so he could help with the rent despite his age.
Sometime during the Buddha Mission, Ayumu awoke to find that his brother was gone. He went outside looking for him but gave up sometime around when Kato died (not to Ayumu's knowledge) although he may have gone back to live with his aunt. When Kato was revived, Ayumu saw his older brother for the first time in months. They go back to living their old lives again. During the Nurarihyon mission, Ayumu awakens yet again to find that his brother is gone. He wanders around for a while outside looking for him again and starts praying that his brother would be returned. Kato survives the mission and they go home together.
Kato is seen protecting Ayumu and other children during the Katastrophe. Ayumu hasn't been seen much besides these few scenarios.