Koichi Aizawa is Rokujou's seemingly 14-year-old classmate who studies Nindō under Kumohira. He is normally portrayed with an energetic attitude around other people, becoming serious and relentless when it comes to ninja duties. He is fascinated with death as a result of his immortality. His immortality is attributed to the holder of the Shinra Banshou during the Edo Period, who was a medical practitioner who experimented with placing the wisdom of the Shinra Banshou into animals, since humans usually died. He and Shijima Kurookano are the only survivors of the experiments and gained immortality; they believe that their mission is to observe the end of Nabari. This implies that Koichi, and subsequently Shijima, are at least a century old. Koichi's ultimate goal to have the Shinra Banshou release him from immortality and ensure that another immortal being or "monster" like himself is never created again. They are together referred to as the Hakutaku. The Aizawa household are descended from Fuuma ninjas and are native Banten ninja. Throughout the series, it is hinted that Koichi has a romantic inclination towards Raimei Shimizu, though the girl has mentioned that Koichi "is not her type of guy" early in the series.
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