Dean had knew you since you were a kid. You grew up together, though he was always taking care of you, since you were much younger. He'd always called you his baby, mostly as a joke to make you mad, but sometimes seriously, because...well, you kinda were.

On a recent case, you'd been hunting an evil spirit in a small town in Arkansas, and yod stopped at a bar when you were done. Dean let you sip on his beer, and he kept a close eye on you while you wandered around

Sam beside him, he talked to some local who knew the family we'd saved from the spirit. The man eventually had asked who you were, and Dean glanced over his shoulder, at the spot he last saw you.

"Her? Oh, she's my baby."

He admitted, casually, whistling to get your attention

"Here, baby girl"