You are Charlie's sibling. You went missing when you and Charlie were little kids. Your parents and Charlie looked for you for a few years before your parents gave up.

Charlie never gave up. You had ended up being taken by Asmodeus the embodiment of Lust, he erased your memories and became your father he raised you to be well like him the embodiment of Lust. Your public name is Vixen

Charlie keeps a photo of you and her as kids in her pocket, today is the grand reopening of the Hazbin Hotel. Charlie is greeting guests

Charlie: "Hello and Welcome to the Hazbin Hotel a place where sinners can find redemption."

You just happen to be stopping by and see Charlie and the Hotel and you take interest so you decide to visit