Hidenori Sakuma, a convicted bomber incarcerated for attempted arson before the Gibia virus outbreak, belongs to the Galients outlaw group in post-apocalyptic Japan. He forged a strong bond with Galients leader Renjiro Hatonami during their imprisonment, becoming Renjiro's "little brother" due to Renjiro's strength and leadership. This bond led Sakuma to follow Renjiro after their escape during a Gibia attack.

Physically short and overweight with brown hair, Sakuma typically wears blue glasses and a green lumberjack shirt. His primary skill is constructing bombs and explosives, used by the Galients in combat. He sometimes attaches these explosives to the hook weapon of fellow member Katsunori Hamuro to extend their range.

Within the Galients, Sakuma handles routine tasks like meal preparation. His repeated cooking of katsu curry drew complaints from other members, revealing his limited cooking repertoire. During food scarcity, he participated in the group's initial ambush of Sensui Kanzaki's group to seize supplies, where Renjiro was defeated. Sakuma later witnessed Sensui's group effectively combating Gibia monsters in a tunnel, visibly impressed by their effectiveness.

The Galients subsequently tracked Sensui's group for another confrontation. A Meteora monster attacked both groups during this encounter, prompting Sensui's group to intervene and save the Galients, leading to an uneasy cessation of hostilities. Sakuma's name reflects his character: "Hidenori" combines the characters for "excellent, outstanding" (秀) and "rule, ceremony" (典), while "Sakuma" signifies "help, aid" (佐), "long time" (久), and "space, interval" (間).