The charismatic leader of the "Five Leaves." A mysterious person, he dislikes telling about his own past and lives in a local brothel, protecting and flirting with the women working in there. Distinguishable by his light colored hair and his tanned skin, he is often seen smoking a pipe. At the beginning of the series he is calm and carefree, seemingly living life to the fullest and intent on living in only the present, as he says to Masanosuke. He even does him a favor by recruiting Masanosuke into the Five Leaves gang at the point where he is at his weakest socially (because he is very shy) and financially. His reasons for befriending the samurai at first was because he was so naive, but over the course of the series Yaichi has a sort of falling out with him because Masanosuke was "annoying." Yaichi eventually becomes the subject of Masanosuke's personal investigation of his past, something Yaichi has long repressed.

It is revealed through flashbacks that Yaichi was actually born as "Seinoshin," a young child who was adopted into a prominent Japanese family, but rejected by his adoptive parents. As a result, he became quiet and withdrawn, with a family servant named Yaichi and Yagi, another heir from another family, as his only friends. He was kidnapped by a bandit named Jin. He believed that Yaichi will protect him but was told that Yaichi was the one who arranged his kidnapping, which traumatizes Seinoshin. When he grew older, Seinoshin worked under the moniker of "Sei," a bandit and merciless killer who during this time (under implication) killed his old servant Yaichi, dumping him in a well for betraying him. But then for reasons unknown Sei betrayed his old gang by selling them out, and fled to Edo where he became "Ichi," forming the Five Leaves gang with a woman named Otake, a woman he paid out of bondage from a brothel. It is later revealed that the original Yaichi did not betray him, forcing Ichi to come to terms with his past.