Morgana is Holly's mother, a vampire, and a former classmate of Count Von Horrificus at the School for Little Vampires. She courted a mortal man decades ago and had a child (Holly) with him. When he found out that Morgana was a vampire, he chased her and their child out of his life.
It is not known how Morgana kept what she was a secret long enough to have a child with the unnamed human man, but it is believed he may have been really oblivious.
Count Von Horrificus is implied to have had a crush on Morgana, at some point. Possibly still does.