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Sanosuke Sagara AKA Sano is the Zanbato weilding, hand to hand fighting, former Sekihotai member who is Kenshin's friend.
Backstory
Sanosuke Sagara, originally born Sanosuke Higashidani, took on the name Sagara from Captain Souzo Sagara, leader of the Sekihou army. The army was originally founded to aid the Revolutionary government during the Bakumatsu Revolution, however as the new government ran into financial troubles, they turned on the Sekihou army, killing most of the members and even executing Captain Sagara. After seeing his beloved role model's head on a pike, Sanosuke then became a fighter for hire, eventually developing the nickname "Zanza the fight merchant" (apparently the name is partly derived from his only weapon other than his fists - an extremely large sword called a Zanbato)
Start of the Manga
When we first see Sanosuke in the main storyline, he is roaming the streets of Tokyo in search of anyone that might give him a challenge in a fight. After some drunks cause trouble in the Akabeko, he sets them straight by fighting the strongest of the three and defeating him in one blow. Right after, he introduces himself to Kenshin and company, who were bystanders during the incident. Later that day, Sanosuke is hired by the Hiruma brothers (Kihei and Gohei) to fight Kenshin, but upon discovering Kenshin was the legendary Battousai of the Ishin Shishi, he decides to fight him of his own volition. In the fight, Kenshin convinces him to become a protector of the weak as was the Sekihou Army's original intention, and from then on he joins Himura and company at the Kamiya dojo and helping with Kenshin's subsequent jobs.