Mark Lilly's mother has some similar features to her son, like brown hair and white skin. She is first seen in the episode "Trolling for Terror" where she apparently has got very intimate with Clark Dungaree.
Mark had a dream of his mother in crotchless panties in an orgy in "Soul Sucker". When Callie Maggotbone's puzzle box makes the dream come true, Mark's mother talks in Callie's voice. As it turns out, she is in fact Callie who has shapeshifted herself to take the form of Mark's mother in the dream re-enactment.