Genryūsai Shigekuni Yamamoto founded the Gotei 13 roughly 1,000 years ago and served as its Captain-Commander. Centuries earlier, approximately 2,100 years before the main events, he established the Genryū and Genji School, the precursor to the Shin'ō Academy. As head instructor, he personally trained future captains Shunsui Kyōraku and Jūshirō Ukitake, later expressing pride in their accomplishments. During this era, young Chōjirō Sasakibe repeatedly sought him out, addressing him as "Eijisai-dono" due to a scar on Yamamoto's forehead resembling the character "丿" (ei). Sasakibe declared his intent to become Yamamoto's right arm, not his disciple, leading to a clash where Sasakibe unleashed his Bankai, Kōkō Gonryō Rikyū. This attack inflicted a second scar, crossing the first to form an "X". The resulting mark prompted others to call Yamamoto "Jūjisai" (十字斎), but Sasakibe's refusal to use the new name led Yamamoto to adopt "Genryūsai" – a moniker reflecting his role as founder ("Gen-" meaning origin) and incorporating cultural/spiritual connotations related to time and the willow tree. When forming the Gotei 13, Yamamoto recruited the notorious criminal Yachiru Unohana as a founding captain. Around the same period, he confronted Quincy emperor Yhwach, leading the original captains and Chōjirō Sasakibe. Though Yamamoto defeated Yhwach and eliminated Quincy elites Seydlitz, Huburt, and Algora, he failed to land a final blow. Yhwach characterized Yamamoto from this period as viewing humans and subordinates as expendable. Post-conflict, Yamamoto embraced a code of honor and justice, abandoning indiscriminate sacrifice. Later, he offered Sajin Komamura a Gotei 13 position despite Komamura's non-human origins and intervened in Kenpachi Zaraki's captain training. Central 46 halted this training fearing Zaraki's potential rebellion. Yamamoto's aged appearance reflects his immense seniority as the oldest Gotei 13 captain. He presents as an elderly man with a bald head, deep wrinkles, and a long white beard secured by a purple band. Prominent scars mark his body, notably the crossed scars above his right eye from Sasakibe's attack. He wears the standard Shinigami uniform with his captain's haori draped loosely over his shoulders. Despite projecting frailty, he possesses a muscular physique only revealed in combat. He lost his left arm during a confrontation with Sōsuke Aizen. In youth, he had black side hair tied in a topknot, thick eyebrows, a mustache, and wore a sleeveless haori over his Shinigami garb. Yamamoto embodies unwavering conviction and strict adherence to Soul Society's laws, demanding the same from others. He exhibits zero tolerance for insubordination and reacts aggressively to betrayal, often addressing opponents as "youngster" and framing battles as "punishments." In peacetime, he projects frailty—even sleeping while standing—and rarely shows distress, typically conveying seriousness by slightly opening his eyes. His loyalty to Soul Society is absolute; he willingly sacrificed himself and the Gotei 13 to stop Aizen, viewing it as duty. He deeply values the captain's haori as a symbol of rank, becoming visibly angered when captains like Kyōraku or Byakuya lose theirs without remorse. Despite sternness, he displays nuanced compassion: he hesitated to attack the childlike Wonderweiss Margela, expressed pity for Wonderweiss's diminished mind, and commended the Tres Bestias for their bravery before defeating them non-lethally. His fury at Driscoll Berci for using Sasakibe's stolen Bankai underscored profound respect for his lieutenant. Culturally, he prefers Eastern traditions, mastering the Japanese tea ceremony and hosting monthly sessions for his division, while disliking Western customs favored by Sasakibe. He maintains personal rituals like monthly beard braiding and sunbathing while being rubbed with a dry towel. Yamamoto's combat prowess stems from over two millennia mastering Shinigami arts. His Zanpakutō, Ryūjin Jakka, is the oldest and among the most powerful in Soul Society. Sealed as a wooden staff, it transforms into a katana upon release. Its Shikai command, "All Things in the Universe, Turn to Ashes," unleashes flames incinerating anything touched, detectable miles away. Techniques include Jōkaku Enjō (trapping targets in spherical firewalls); Taimatsu (precise wide-area incineration); and Ennetsu Jigoku (a high-risk pillar-based attack also endangering Yamamoto). His Bankai, Zanka no Tachi, seals all flames into a blackened blade, raising ambient temperatures lethally. It manifests four forms: Higashi: Kyokujitsujin (erasing anything the blade touches); Nishi: Zanjitsu Gokui (coating Yamamoto in invisible 15,000,000-degree defensive flames); Minami: Kaka Jūmanokushi Daisōjin (summoning charred remains of Ryūjin Jakka's victims to overwhelm foes psychologically); and Kita: Tenchi Kaijin (unleashing a concentrated slash reducing targets to ash). Beyond his Zanpakutō, he excels in Hakuda, using techniques like Ikkotsu (a single punch obliterating Wonderweiss' abdomen) and Sōkotsu (a two-fisted strike). He is also proficient in Flash Step and Kidō. Key events later in his tenure include overseeing the Gotei 13 during the Turn Back the Pendulum arc (110 years prior), initiating Kisuke Urahara's captain promotion and managing disappearances linked to Hollowfication experiments. During Sōsuke Aizen's betrayal, he sacrificed his left arm to perform Ittō Kasō, imprisoned Aizen's faction with Jōkaku Enjō, and fought Wonderweiss, culminating in Ennetsu Jigoku. In the Thousand-Year Blood War arc, after Sasakibe's death, he executed Driscoll with Ryūjin Jakka for wielding Sasakibe's stolen Bankai. Confronting Yhwach's impersonator, Royd Lloyd, he activated his full Bankai, revealing all four forms before killing Royd. The real Yhwach then appeared, exploiting Yamamoto's emotional turmoil over fallen comrades and bisecting him. Yhwach criticized Yamamoto's shift from ruthless pragmatism to honorable restraint, obliterating his remains. Posthumously, Yamamoto gained the epithet "Founder of the Gotei" upon being cast into Hell.