Raizo was born into a poor village in the district of Zaraki in Rukongai. His village was a part of a resistance effort against an organized crime empire, the Yonshetou outside of Seireitei. This effort was led by a man named Tetsuya Sajin. In a personal hit against Sajin, the Yonshetou massacred the village. Raizo was the only survivor. As a young boy, Raizo learned to fight, kill, and engage in espionage to help his adopted father, who became his mentor, protector, and the most feared scourge of Soul Society's criminal underworld. When Sajin acquired his Zanpakuto, the two were able to crush the Yonshetou in retribution for their atrocities. After this, Sajin was conscripted by force into the Goteijuusantai.
His departure was sudden and without notice. For nearly one hundred and seventy years, Raizo was left without closure. Based on what he knew, it seemed that Sajin had left without telling Raizo, and didn't care about his role as a father anymore. After a while of simple hunting and living, Raizo began realizing he had his own reserves of spiritual power. Recognizing his chance to find Sajin, he eagerly enlisted in the ranks of the Shinigami. However, by that time, Sajin had become insane, imprisoned in the Maggots' Nest. Raizo would be denied his goal for another several decades.
Finally, Raizo encountered his father once more. During a Hollow hunting assignment, Sajin, who had returned to Soul Society searching for Raizo, tracked the new Shinigami down. Stripped of most of his former power, but now a treasure mine of information, skill and techniques, Sajin mentored Raizo for a long year. That year's worth of training was disguised as a strange request of Sajin's. The former Shinigami claimed that he was dying of a drainage of spiritual energy, and wanted a worthy mercy killing from Raizo. But, Sajin's true intent was to bring out Raizo's full power. After his long leave from Seireitei, Raizo discovered a number of documents in Sajin's safehouse. These detailed missions, questions and investigations that Sajin had charged Raizo with the resolution of. Adopting the house as his own, Raizo prepared himself for the coming years, in which he would take on many missions to the Human World as an excuse to fulfill Sajin's directions.
Over that time, he became disillusioned with Soul Society, taking a leave of absence in the Gense. After a few decades, he returned to the Tenth Division for the sake of Rukongai. By advancing through the ranks, he acquired power and wealth that he turned to reinvigorating the society. At this point, word of his name spread through Rukongai, prompting the Konoha Clan to find him and appoint him the last head of the Shinoda Clan, a noble house of which Raizo was the remaining scion. He rallied them together against the Yamata no Orochi, a remnant group of the Yonshetou's more criminally insane members. Together, they defeated the Yamata and thereby saed Rukongai, prevented an attack on Seireitei, restored the Shinoda name, and set the foundation for the Order of Assassins. During this, he met a Konoha woman of similar age and fierce spirit named Asami Shinoda. The two developed a mutual admiration for each other which eventually developing into romantic love. At the end of the Yamata conflict, the two were wed.
Several years later, Soul Society was attacked by an Arrancar named Oscurra Valtus. A fusion of former Tenth Division Third Seat Valter Kyladislas and Privaron Devarus Oscurro, she proved to be an overwhelming adversary due to her unique Resurreccion abilities and tremendous physical strength. Oscurra's plan was to assembly a vast army of souls by controlling them with her Resurreccion. Then she would destroy Seireitei. Raizo was able to repel her by drawing out their first encounter outside the Tenth Division that she dared not stay, lest a Captain arrive. She did manage to seriously injure Asami, who was pregnant at the time, and damage her mind to the point where she needed to be sedated. Raizo pursued Oscurra with the Assassins in tow. Along the way, he learned to master his Bankai abilities to a finer degree. Upon catching up with her in the Zaraki District, he fought her without restraint and lost. Oscurra placed him under her mind-controlling spell and left him to recover while she went to gather more souls. This proved to be her undoing, however. Raizo was able to throw off the mind control unbeknownst to her. With his powers recovered, he chased her down and defeated her. However, he found himself unwilling to kill her. So Raizo imprisoned her in a bunker he had built under the Namikaze farm. When he returned to Seireitei, however, he found that his wife had died of her injuries. His children hadn't survived either. He retreated to the Namikaze farm and isolated himself in sorrow.
