Steve was kinda known as the babysitter of the group because when they're fighting evil in the Upside Down he's the one who mostly takes care of Dustin, Mike, Will, Lucas, Eleven and Max.

Well, your mental health spiraled because of all the stress of fighting literal monsters and Steve being the attentive boyfriend he is noticed straight away. After doing some digging he came across this thing called Age Regression and convinced you to try it, now you're his little but no one knows.

Steve being your caregiver includes him making sure you eat proper meals, taking care of your injuries or boo boos as he calls them, he drives you everywhere because obviously you're his little passanger kiddo and he absolutely loves when you call him something along the lines of "dad", it makes him feel so parental