Aoshi is the Okashira of the Onmitsu Oniwabanshu(the leader of the Oniwabanshu, the protectors of the Bakufu castle). He was an oniwabanshu as long as he remembers, he became the Okashira at the age of 15 (when the former Okashira -Misao's grandfather-died). He promised his former okashira that he would make the Oniwabanshu mantain their honor and that he would take care of his grandaughter (Makimachi Misao).
To gain the respect of the elder oniwabanshu, he hid his emotions to the point where he didn't show any. He never lets anyone know his emotions, and his face shows no kind of expressions. He can appear to be cold, indifferent, and merciless at times.
The only time he looked kind and gentle was when he was with Makimachi Misao (the former Okashira's grandaughter). He helped to raise her and trained her (with the help of the other Oniwabanshu).
When the Oniwabanshu was disbanded he was offered many jobs in the goverment, but he denied them because he could only accept and offer that included the other Oniwabanshu. After that he took the Tokyo Oniwabanshu (Beshimi,Shiki-jou,Hyottoko, and Hanya) to go around and work as bodyguards (since fighting is all they knew to do).
He got involved with a opium dealer (because of a debt of honor) and worked for him. He battled Kenshin for the title of the strongest warrior (for the honor of the Oniwabanshu), but he lost.
Then his employer (who hated the Oniwabanshu because they didn't follow his orders instead of Aoshi's) tried to kill him with a gattling gun. The Tokyo Oniwabanshu gave their lives to save him, and with their help Kenshin defeated the madman. He took their bodies and made graves for them with his hands and his saya.
He went into hiding, until he got involved with Shshio. There he almost killed the current Okashira (Okina) and confronted Misao from the first time since he left with the other Oniwabanshu. He told her that he never wanted to see her again (he said that because he didn't want to hurt her or so that she'd see what he had becomed).
The Aoshii Misao Shrine Ver. 2.0