Marian Pewterschmidt is Patrick Pewterschmidt's imaginary wife who supposedly appears in "The Fat Guy Strangler".
Brian and Stewie Griffin placed a cucumber on the couch where her vagina should be and bet that if after three weeks it pickles, then Marian is real and they'd both have to buy Patrick a steak. Her sister-in-law Lois later notices it, asking who left the pickle on the couch, sparking minor debate over whether or not she is real.
While in James Woods, Lois threatens to decapitate and kill her while confronting Patrick in the forest. Because she is invisible, her police sketch is blank.