Yoichi Saotome is a central character, first introduced as a student striving to join the Japanese Imperial Demon Army's Moon Demon Company. His drive originates from a traumatic childhood event during the apocalyptic outbreak, where he witnessed his older sister, Tomoe Saotome, killed by the vampire Lacus Welt while hiding under a bed on her instruction. This experience forged a deep desire for vengeance, though his initial attempts to enlist were met with rejection. In appearance, Yoichi is of average height with medium-length brown hair and large, dark green eyes. His look varies slightly between adaptations, with spikier, thinner hair in the manga. He is first seen wearing a gray and light blue gakuran before adopting the standard black long-sleeved coat of the Demon Army, which includes light green details, a brown Sam Browne belt, gold buttons, white gloves, a white web belt, black trousers, and boots. He often accessorizes with a cap and cape. His personality is marked by kindness, altruism, and optimism, making him a natural peacemaker within his squad who often mediates disputes between comrades like Yuichiro Hyakuya and Shihō Kimizuki. This gentle demeanor initially helped him maintain mental stability against his demon. Yet, a darker, vengeful side surfaces in private exchanges with his demon, Gekkouin, where he voices a desire to inflict suffering upon his sister's killer before death—an aspect he deliberately conceals from his friends. Yoichi’s background includes being one of many children subjected to experiments by the Hyakuya Sect as part of the 'Seraph of the End' plan, a fact unknown to him but known to Guren Ichinose. Before enlisting, he endured bullying from other students at Second Shibuya High School, persevering for another chance to take the army exam. His story advances when he aids Yuichiro Hyakuya against an escaped vampire, an act that secures both their acceptances into the army and placement in Shinoa Hīragi's squad. He eventually forms a contract with the demon Gekkouin, which manifests as a black bow from the Black Demon Series after he initially fails the trial. This cursed gear, named Gekkouin (Moonlight Rhyme), enables long-range combat, multiple arrow manifestation, and enhanced targeting, with a green pentagram symbol shielding his left eye during use. As a soldier, Yoichi takes part in critical operations such as liberating humans from a vampire lair, defending Shinjuku from invasion, and providing aerial support during the Nagoya raid against vampire nobles. He collaborates with figures like Shinya Hīragi and Makoto Narumi's squad. His skills and confidence grow markedly over time, evidenced by his solo sniping of multiple Horsemen of John and his acquisition of advanced sniper techniques from Shinya Hīragi. A pivotal moment occurs when he assists Mikaela Hyakuya's escape and, horrified by Kureto Hīragi's sacrifice of soldiers, fires upon him, resulting in his desertion from the army alongside his squad. He is compelled to cooperate with the vampires Ferid Bathory and Crowley Eusford, during which he learns of Guren Ichinose's scheme to resurrect all who perished in the apocalypse. In Shibuya, he helps Kimizuki rescue his sister, Mirai, and becomes enraged upon discovering Guren has preserved his own sister Tomoe's body. Despite reservations about Guren's methods, he takes part in executing the plan. Yoichi exhibits a high natural resistance to psychic and demonic energy, which enabled him to endure Guren's testing for the Black Demon Series unaffected. He also demonstrated the rare ability to manifest his demon without formal training and possesses notable physical strength, enough to carry comrades heavier than himself. His relationships are defined by profound loyalty to his squad, which he regards as a new family. This bond is central to his personal growth, helping him overcome nervousness and discover greater self-worth and purpose beyond revenge.