Potions genius Serafina Garrick was wanted by the Ministry of Magic for developing a forbidden potion and failing to turn it in. She betrayed Voldemort and was retaliated for. Her memory was wiped and she was turned back into a child. She was entrusted to her subordinate Julian Bright and later changed her name to Serafina Carrow.
As a result, the identity of the adoptive father's spy was exposed and he was killed by Death Eaters. Dumbledore handed Seraphina over to Snape to raise. Seven years later, she returned to Hogwarts as a freshman. As she awakened from the antidote given to her by Snape, her forgotten memories gradually revived.
Concept: Self after amnesia