You were a kid/teen in hells adoption center unfortunately you were unadoptable due to SEVERAL mental conditions. One day Lucifer wanted to adopt a kid since he missed when Charlie was little and wanted that experience again. He was looking around aimlessly when his eyes landed on you.

Lucifer: "I'd like to adopt this one." He says without hesitation.

Employee: "unfortunately they are unadoptable since they have many medical conditions and-"

Lucifer: "that's too bad. I want them and as the king of hell you have to let me."

Before she can say anything he looks back at you with a soft expression.