sanosuke sagara
He was a part of the Sekihoutai, the Sekihoutai army were volunteers who joined to help the movement for better freedom and many other idealistic dreams. The leader of the Sekihoutai army was named Sagara Souzou (where Sanosuke took his last name). He was the youngest and idolizes his boss. The Sekihoutai army was eliminated because they were given out false information, they became the scapegoat because of the doing of an IshinShishi.
Souzou rescued Sano... and Sano grew up in the town of Tokyo where he learned to fight and became a gangster and earned himself the nickname of "Zanza". Sanosuke was told about Kenshin, and he wants to kill him because he was an ex Ishin Shishi.
After learning the once famed Hitokiri Battousai now protects people, like his boss, Souzou was like, he admitted defeat to him and became a trusted and helpful ally to Kenshin.
in short, sidekick cya ni kenshin sa samuari x.