Sagara Sanosuke (相楽 左之助?) is a former member of the Sekihotai. When the group is destroyed by the Meiji Government, he becomes a fighter-for-hire to calm his anger by fighting. During his introduction in the series, he encounters the wanderer Himura Kenshin, who easily defeats him and is able to convince him to stop his mercenary work and instead start protecting people. After that encounter, Sanosuke becomes Kenshin's best friend as well as his partner in most of their fights.
The destruction of the Sekihotai and the loss of all his friends has caused Sanosuke to hate the Meiji Government and he wears the Japanese kanji for "evil" (惡 aku?) on the back of his jacket. He bears this symbol as a mark of his loyalty towards his past in the Sekihotai, but it also serves as a reminder that he will defend innocent people who have been wronged so others can avoid the same suffering he experienced. Sanosuke's influence on other characters is demonstrated when they place the same symbol of the Japanese kanji on their backs, as does his younger brother Higashidani Ota.
Despite hating the Meiji Government, Sanosuke becomes Kenshin's best friend and sees him as a gentle person. Sanosuke compares Kenshin to his former captain, Sagara Sōzō, whom he admires. Although Sanosuke may punch him occasionally for preventing him from fighting, Kenshin notes this makes Sanosuke is his most reliable friend, always willing to help.
At the start of the series, Sanosuke uses an enormous zanbatō in battle, giving him the moniker Zanza (斬左?). The blade has no edge and Sanosuke only uses it to smash and crush his opponents. After being defeated by Kenshin, he stops using the zanbatō and decides to focus on hand-to-hand combat. After that, Sanosuke is introduced as Kenshin's most powerful ally, having gained superhuman strength and toughness from the numerous battles he has experienced since the age of ten. Within his trip to Kyoto, Sanosuke learns the Futae no Kiwami (二重の極み? lit. "Mastery of the Two Layers") technique from a fallen monk named Yūkyūzan Anji. Futae no Kiwami is a special art in which the user performs two hits in quick succession with any part of his body. Sanosuke is only able to perform it with his right hand; the first punch with the proximal interphalangeal joint neutralizes the hardness of the target, then the second blow, struck with the proximal phalanx, breaks the target before it recovers from the force of the first blow. Sanosuke's right hand is severely damaged from the overuse of this technique, and this becomes a major hindrance to his fighting ability during the Jinchū arc. He overcomes this disability when he discovers that the Futae no Kiwami can be performed using both hands in succession which lessens the impact to his damaged hand.