A former comrade of Gintoki and Katsura and a major antagonist. He now leads a radical Joui faction called the Kiheitai and is known amongst the Shinsengumi as 'the most dangerous man in both the Joui faction and amongst the radicals.' He is easily recognizable through his bandaged left eye, his colorful kimono, his eerie smile, and the pipe he is often seen smoking. He, Gintoki, and Katsura studied in the same temple elementary school when they were children, and he fought with them against the Amanto invasion during the latter part of the Joui war. After the war ended, he vanished, and has since gained a dangerous reputation due to his assassination of many of the Bakufu's main officers and his planning of a large-scale coup d'etat. Before his first appearance in both the anime and the manga, he was mentioned to have fled Edo and to have been hiding in Kyoto, although it is unclear how long before this was. He now believes that the only way to save it is to destroy the "rotten" post-Joui war world. He is shown to like festivals, although this is mainly due to the opportunity they provide for a large-scale terrorist attack: he appears to have an attraction to ruin that is almost nihilistic. He also appears to have considerable insight due to his 'seeing through Itou's heart' and subsequently planting him in the Shinsengumi's ranks. It can be assumed that he is as skilled a swordsman as both Gintoki and Katsura, having fought alongside them as one of the 'heroes' of the Joui war. That aside, he appears to have at least considerable skill in playing the shamisen, and is shown at one point playing it with Bansai as they talk. He is based on the historical Takasugi Shinsaku.

He plays percussion and is the leader of Blast, usually handling the business partnerships and financial decisions needed for the band's success. His favorite cigarette brand is "Black Stones", from which the band got their name. He is also a lawyer in training, despite his "scary" appearance (clean shaven head, many earrings and dark glasses). He's the big brother figure for Nana O—and sometimes Nobu—and acted as Nana's "guardian" when she arrived in Tokyo. While he harbors some feelings for her, he baits Ren to rekindle their love for each other. In the past, he shared several ties with Trapnest's members. Yasu and Ren grew up in the same orphanage and remained close friends when Yasu was adopted. He dated Reira while in high school and was jealous of Takumi's success and influence on her, which he later relates as the reasons for their break up. Also, he's the oldest member of Blast which Nobu sometimes teases him about (by calling him "old man"). He is in a budding romance with Miu to the surprise of Ren and others—who were sure he would date Reira again.