Mark Foster is a hero from the 1991 television series Step By Step. He looks out and stands up to threats to his blended family. Mark is a Caucasian male with blond curly hair. His signature accessory is his round glasses. His usual outfit is a conservative dress shirt overlaid with a sweater vest. He begins the show as a meek & nerdy ten year old boy. At the end of the show, Mark has grown into an assertive & well-rounded seventeen year old. He is the youngest member of the Foster side of the family. He is portrayed by Christopher Castile.