Kyūju Amagiri serves the Satsuma Domain as a demon, distinguished by long red hair tied in a ponytail, cyan eyes, and a goatee. He presents as a tall, bulky middle-aged man, typically attired in traditional dark-purple trousers, a red-vine-patterned top, a black haori featuring a white snake forming an infinity symbol on the shoulders, and long gloves. Functioning primarily as an advisor and companion to Chikage Kazama, Amagiri acts as a voice of restraint when Kazama's actions risk shaming Satsuma or violating the Demon's Path. He maintains a consistently polite and respectful demeanor toward allies and adversaries alike, acknowledging opponents' strengths and offering gratitude or apologies as warranted. Amagiri avoids unnecessary conflict, preferring verbal resolution and engaging in combat only when compelled. He fights using his fists, aiming to incapacitate opponents efficiently with minimal strikes to deter further aggression. During the Ikedaya incident, Amagiri confronts Heisuke Tōdō, urging him to avoid a fight before knocking him unconscious when Heisuke insists. Later, during an ambush by Satsuma-Chōshū forces, he proposes sparing Shinsengumi members in exchange for surrendering Chizuru Yukimura. After this offer is refused, he engages Heisuke again, inflicting grave injuries that force Heisuke to become a Rasetsu to survive. In subsequent events, he intervenes to halt battles between Kazama and Toshizō Hijikata, emphasizing the need to retreat to uphold Satsuma's honor. In later conflicts involving Kōdō Yukimura's daytime Rasetsu army, Amagiri reveals critical information to Hajime Saitō about the Rasetsu's shortened lifespans, explaining that each use of their powers consumes life force. He advises Saitō to warn other Shinsengumi affected by the Ochimizu elixir. He continues to counsel Kazama, cautioning him against emotional involvement with humans during a confrontation with Hijikata. When Kazama ignores this advice, Amagiri withdraws his support, stating Hijikata may deal with Kazama as he sees fit. In the "Sweet School Life" OVA, Amagiri appears briefly as a student at Hakuou Academy alongside Kazama and Shiranui. His background lacks a historical prototype, and no significant character evolution or deviations from his established role and demeanor are documented across media.