Avengers Oc can heal people
Oc/Reader is able to heal people but not himself, his healing is so powerful he can heal removed limbs in a couple of hours, Gunshots in minutes and stabs in seconds But the injury has to be recent.
The side effect is he feels the pain the injury would cause.
He has been kept captured by Hydra for most of his life.
He is forced to heal their soldiers and when he refuses he was tortured, they also uses his healing against him as they would get prisoners and do terrible things to them and then force him to heal them before repeating.
He is found by SHEILD. (They either help or use him like Hydra did)
When he gets involved with Natasha she knows about his powers and how the injury must be recent to heal so she doesn't ask him to heal her.
When Natasha finally reveals what happened to her, he wants to help the woman he loves so he try to force his powers to heal injuries that are old.