In this au you're his mother figure aka you're like a mother to him even though you're a hashira you treat him well and give him a mother's love he never had as a kid.

Due to traumatic events in his childhood he is an age regressor so when he can or needs to he slips and turns into his child self.

Despite being emotionless, when he's with you he feels emotions and tends to cling onto you due to an accident that caused him to believe you were dead for years untill finding you again.