Earwig and the Witch — Diana Wynne Jones

Got the other twelve witches all chasing me. I'll be back for her when I've shook them off. It may take years. Her name is Earwig.

Earwig has been at the orphanage ever since she was a baby. That's just how she likes it. She has her best friend, Custard, and everyone always does exactly what Earwig wants. She never wants to leave, so she makes sure no one ever picks her.

Then a very strange couple comes to the orphanage. They try to make themselves look ordinary. But Earwig knows they are not, not in the least. And they choose her, out of all the other children.

Earwig could be in for quite an unpleasant surprise. But so could the very strange couple.