Background:

The Oniwabanshuu were a group of ninjas who protected the Edo Castle. At age 15, Shinomori Aoshi was chosen by Okina to be their Okashira after the death of their former leader. Misao’s grandfather had chose Okina to be his successor since his skills matched that of his own, but Okina thought it would be better if a younger generation was to lead them.

Aoshi sealed his emotions thinking it would only get in the way and hold him back from becoming a great leader, in which people look up to.

Much to his regret, Aoshi did not fight during the Bakumatsu no Douran (turmoil), because the battle had ended before it reached the Edo Castle, where he was stationed. The Oniwabanshuu lost their chance in proving their strength.

After the war ended, many of the Oniwabanshuu were given governmental positions and returned to living normal lives. Aoshi turned down his offerings and chose to remain with the Oniwabanshuu, knowing some, like Hannya, Shikijo, Beshimi, and Hyottoko were unable to merge back into society. He felt he was responsible for them.

Rurouni Kenshin Series:

We first meet Aoshi and half of the Oniwabanshuu at the end of manga volume 3 and episode 8 in the anime. He was working as body guards for Takeda Kanryuu, a greedy businessman involved with illegal activities. Kanryuu used Megumi to make opium and sell them to common people. He meets the Kenshin (Battousai) and his friends at Kamiya dojo, when chasing after Megumi. Aoshi challenges Kenshin to a match for the title of the strongest. He wanted to prove that the Oniwabanshuu was the strongest. At first, Kenshin was overwhelmed by the strength of Aoshi's techniques - using a combination of kodachi and kempo. Aoshi lost the fight and wanted Kenshin to kill him or he will come after him.

Kanryuu shows up with a gattling gun aimed at Hannya, but Aoshi knocked him aside and got wounded. His comrades manage to protect their injured leader, with their lives. Aoshi's sanity breaks and becomes more obsessed with the idea of winning the title of strongest for his people. He buries his comrades in the mountains and promises to give them peace once he can get the title of the strongest.

At the beginning of the Kyoto arc, Aoshi shows himself once again, entering the Kamiya Dojo wanting to have a rematch with Kenshin. Unfortunately, Kenshin had already left for Kyoto and Megumi accidentally runs into him. Aoshi threatens to kill her if she didn’t tell him Kenshin’s whereabouts. Saitou arrives and informs Aoshi where Kenshin was. Although Saitou wasn’t interested in helping Aoshi, he felt that Aoshi will be of some use.

Eventually, Aoshi comes to an alliance with Shishio Makoto. He wanted Okina to tell him the whereabouts of Kenshin, but since he refused, he attempted to assassinate Okina and betrays his own people. Towards the end, Aoshi got his rematch with Kenshin. This time, Aoshi uses kodachi nitou. Kenshin’s lectures awoken Aoshi from his ambitions and realized that he has lost sight of his original goals. Both opponents used their succession techniques. Aoshi lost and Kenshin continues on his way to Shishio.

Aoshi shows up in battle once again when Kenshin and his comrades have fallen and stalls Kenshin some time. Shishio finally lost and everyone except Saitou, returns to the Aoiya

After Aoshi’s wounds were healed, he started going to the temple daily and meditated, trying to reflect on where he went wrong. In the anime, before Kenshin left to return to Tokyo, he makes a promise to drink tea with him.

Anime:

We meet Aoshi again for a bit during the Christian arc, when Misao goes after the killer and ends up getting wounded. He sends note to call Kenshin to visit them.

Aoshi’s final appearance was episode 89 when he goes to Tokyo with Omasu and Ochika (Okon) to retrieve Misao, who was stayed at the Kamyia Dojo with the Kenshin-gumi. Aoshi fulfills his final promise with Kenshin and has tea with him.

We never see Aoshi use his kodachi’s again.

Manga:

At the end of the Kyoto arc, it led into Jinchuu arc. Kaoru writes a note to Misao asking her to bring Himura Tomoe’s diary from Kyoto to Tokyo, hoping to stop Yukishiro Enishi from his insanity. Okina asked Aoshi to accompany Misao and planing to have tea with Kenshin.

When Aoshi and Misao arrived it was after Kaoru’s funeral. Misao was in shock but Aoshi didn’t believe it and decided to carefully look at the details. Aoshi believed that Kaoru’s corpse was just a puppet, and concluded that Enishi would want to destroy Kenshin by killing Kaoru in from of him. Aoshi points out that there was someone capable of making dolls since he has heard rumors of such a technique. Megumi and everyone else was against him but Yahiko decided it was a chance to take. Aoshi used his Kodachi to cut the corpse and finds that it was indeed a fake body. He finds that Kaoru was still alive somewhere.

Aoshi warns everyone not to tell Kenshin, since he would only blame himself for not protecting her and will be tormented by guilt.

Aoshi goes out and finds Gien (the maker of the puppet), after predicting that he would come back for his doll. Aoshi sets a trap and confronts him in the graveyard at night. Gein was forced to reveal Kaoru’s whereabouts and dies. Saitou finds Aoshi in the graveyard next morning, surprised that Aoshi didn’t die in battle. (His response was stated that he had someone to return to.) Aoshi reaches Enishi on his island and ends up another fighting one of his goons, named Suzaku. He claimed that as his last battle as Oniwabanshuu Okashira.

Aoshi returns to Kamiya Dojo and waits for everyone to recover. Aoshi has tea with Kenshin, as promised then announces he'll return to Kyoto with Misao, planning on taking his friends with him. He wishes to give them their peace and bury them in a warmer area.