Someone who frown-smiles. They either do so because 1) they are not confident enough to smile, or 2) they want to create the impression of modesty and ingenuousness. This latter reason is usually a means to a particular end, for instance frown smiling in order to convince an audience of a statement or belief.
Why does Butthead such a frown smiler?
Devlyn Steele is totally trying to get into our heads with that frown smile.