This is probably one of the few I actually had planned to write before figuring out it was just fucking Yakuza. I swear if this is a common story style I will commit arson.
-basically it's begins with said assassin inside the house of main character, the assassin has a strict moral code not going after people with families to look after. No one told her this time though
-As she's about to leave though main character comes across them, not really realizing that she murdered their parents since they're really young and also stupid.
-Since she's just basically ruined this child's life she decided to look after them as sorta redemption. Even though her kill count is several hundred.
-Years pass, the kid is growing up and finding out more and more about their guardian, but still doesn't realize it was her since, y'know trauma.
-Assassin keeps desperately trying to keep the kid off her trail, since after years of caring for them she sees them as her child. Also it's like, her only chance to try and live a normal human life.
-Kid eventually runs away after finding out she killed their parents. Assassin follows hoping to find some way to convince kid it was necessary. Maybe the parents were assholes, murders, war criminals, doesn't matter she just wants to keep feeling human and the kid is the only way she can.