Sanosuke joined the Seikihou Tai at a young age. It is unknown how he got involved, but Sano looked up to the leader of the group, Sagara Souzou. Although young, Sano was loyal to his leader and group until the Seikihou Tai was forcefully eliminated by the Meiji Government themselves. From that day, when Souzou was executed before a public audience, Sanosuke wandered and wound up in Tokyo.

Throughout his years of growing, Sano became a gangster, beating up people for fun. He went by the name Zanza because he sometimes carried a Zanbatou, a heavy horse cutting sword, with him. On the back of his shirt was the character of 'Aku', meaning evil or bad. When asked by Hiruma Gohei to kill Kenshin, formerly the Hitokiri Battousai, Sanosuke readied his infamous Zanbatou and went off into battle. During the fight, Sano was hyped up to kill Kenshin because he was one who helped create the Meiji Government, but he also found out that what's done is done. The past cannot be changed, therefore, Sanosuke ended up continuing his life with new found friends. Oddly, he found Kenshin to be similar to Souzou in way of thinking.

Often, Sano hangs around the Kamiya Dojo, freeloading off of food and some of Kaoru's money. A favorite place of dining is the Akabeko for Sano. Yet, he had not paid once for a meal, leading to a large tab. Sano was the one who convinced Megumi from committing suicide, and in return, became one of her most frequent patients once he broke his hand in the fight against Shishio.

When he fought Kenshin for the first time, Sanosuke's Zanbatou was broken. Although he later did what he could to repair it, Sano acquired the Futae no Kiwami from Anji in a short learning span of just one week. The downfall is that Sano can only execute the move with his right hand. Before that, Sano relied on basic brawling for his arsenal.

The name Zanza was still feared in the streets of Tokyo when Sanosuke stopped going under that alias. Overall, Sano is a pretty laid back guy who prefers to live his past life. He does complain about the government a lot and has no fear in expressing his hatred for them. Once Kenshin had defeated Enishi, Sano left Tokyo to explore the world, promising to return to the Kamiya Dojo one day. In memory of him, Yahiko wears the 'Aku' symbol on his clothing.