The leader of the Oniwa Banshu, Shinomori Aoshi, became the group's leader at the young age of fifteen. During the ending years of the Tokugawa Era, Aoshi had recruited men who were unwanted and underrated, as he saw them as skilled warriors. Many of the men looked up to their young leader, but when the era ended, many of the Oniwa Banshu members gave up their current lives to move on. Only four men stayed with Aoshi till the bitter end, Hannya, Shikijou, Beshimi, and Hyottoko.

Because of those four, Aoshi had kept on living, as they were the only ones who had not betrayed their master. Aoshi and the Oniwa Banshu were located in Kyoto, but due to little or no pay for warriors in the Meiji Era, the five men moved on to Tokyo where they joined under a wealthy drug dealer named Kanryu. Aoshi had left behind the only one to ever love him, Makimachi Misao, the granddaughter of the leader previous to Aoshi of the Oniwa Banshu.

Always, Aoshi wants to prove himself and his men to be the strongest. In Tokyo, the Oniwa Banshu had gotten a bad name because they were affiliated with Kanryu, yet, Aoshi and his men kept their sense of pride. Kanryu thought he owned the Oniwa Banshu, but Aoshi never showed any signs of being controlled. Aoshi was his own individual and not a lackey. When Kenshin had raided Kanryu's mansion due to the kidnapping of Megumi, the Oniwa Banshu had fought with Kenshin and Sanosuke. In the end, it was Aoshi against Kenshin. This was the biggest fight for Aoshi. If Aoshi could defeat the man known as Battousai, he would earn himself the title of the strongest. Unfortunately, Aoshi was defeated in combat, but promised to fight Kenshin again another day. Shortly after, an enraged Kanryu tried to kill off Aoshi for disobeying him with a gatling gun, but instead, the loyal four to Aoshi protected their master with their bodies.

Aoshi buried Hannya, Shikijou, Beshimi, and Hyottoko's bodies in the mountains and lived there in solitude for quite some time. When word about Shishio Makoto's plot to overthrow the government was heard of and Kenshin went off to Kyoto, that's when Aoshi revealed himself again. Over the period of time, Aoshi had turned into a devil, building up hate for Kenshin because he wanted to earn the title of the strongest for his men. With some help from Soujirou, Aoshi returned to Kyoto in search of Kenshin.

Upon his return to Kyoto, Aoshi was not the same person as before. He had joined an alliance with Shishio, since he was promised a battle with Kenshin in the future, and even went as far as to severly injure the Okina, a well-respected Oniwa Banshu member. In Shishio's base, Kenshin encountered Aoshi. The promise of another battle between the two was kept, but it was different than before. Although the battle had ended in the same way, Aoshi losing, there was a different result. Kenshin had hit Aoshi with the succession technique of the Hiten Mitsurugi Ryu, the Ama Kakeru Ryuu no Hirameki, causing Aoshi to see the light. No longer was he possessed with the goal of becoming the strongest.

Currently, Aoshi lives in Kyoto at the Aoiya, where other Oniwa Banshu members such as Misao and Okina live and work. For a lengthy period, he spends time meditating in a shrine to think about his past, despite being harrassed by Misao at home. Still, Aoshi is a quiet indivdual. He keeps to himself most of the time, as he is that type of person. Never has a smile been seen from Aoshi by anyone. His weapon, the kodachi, is a sword shorter than normal. The kodachi is used for defense, rather than offense. Because of its lighter than normal weight, Aoshi uses it to defend while he mixes his knowledge of kenpo in his fights. Later on, he upgrades and uses two kodachis.