Fan Casting Mysterious Stranger in Sort by Tropes

Some characters first appear with most or all of their identifying traits withheld from the other characters and the audience. They likely won't be greeted by other characters as friend/lover/family/colleague/boss. Their faces and forms are often hidden in shadow or actual disguise. Their very presence may go unnoticed by some or all of the other characters. Yet they keep appearing: the author repeatedly mentions them in the text; the camera focuses on them and follows their actions. Withholding information in this fashion piques the audience's curiosity. Without it, the audience can't form solid expectations about the character.