Hannya is a pupil of Aoshi. The village he was born in was poor, so he was abandoned at birth to reduce the number of people to feed. Hannya survived, but he could not return to the village so he wandered around like an animal until Aoshi found him and gave him a position in the Oniwabanshū.

Hannya is the master of disguise within the group, an ability he gained by removing all his prominent facial features, which is why he wears the demonic mask on his face. As Aoshi gave him something to live for in the Oniwabanshū , he is very loyal to him, he even disfigured his face so that he could easily disguise himself as anyone and gives his life to allow Kenshin enough time to retrieve his sakabatou to defeat Kanryuu and save Aoshi. Hannya's style of combat evolves primarily from his discipline from Aoshi. However, he adds his own bit to it by having painted his arms in horizontal stripes, giving them the appearance of being shorter than they actually are, thus luring opponents into a false sense of security regarding distance of attacks.

(wikipedia)