Please help me.

I'm missing an action story, if anyone knows the title please help me.This about the story of a person who has a skin disease that has no cure, and the only person who always looks after him is his older sister., until it was discovered that the disease he was suffering from actually came from poison, so he could absorb the poison without causing any side effects. He learned this from a woman who became his teacher and taught him how to overcome the disease. If I'm not mistaken, the teacher has red hair and she used to have the same illness as the man and was helped by a doctor,and the teacher is a woman who is on the wanted list .