Well, surely the girl herself is a main character. And probably her family members, like her parents or siblings who would be deeply affected by her vanishing. There might also be a detective or a police officer if it's a mystery - style story, as they would be trying to solve the case of her disappearance.
Well, the 'the girl who vanished story' could be about a lot of things. Maybe it's a mystery where a girl suddenly disappeared without a trace. It could involve her family and friends desperately searching for her, and there might be some strange circumstances around her vanishing, like she was last seen near an abandoned place or with a mysterious person.
In a typical 'family that vanished story', the main characters would be the family members. There might be a father who is a hard - working man, a mother who takes care of the household, and their children with different personalities. Maybe there's an eldest son who is brave and tries to protect his family during the events leading to their disappearance. Or a young daughter who has some special abilities or knowledge that could have been a factor in the mystery.