Dennis is Roger's former partner when they formed a ventriloquism act.
When Steve tries to come up with an idea for a school talent show act in "The Talented Mr. Dingleberry" after his singing is sabotaged, Roger pitches becoming his ventriloquist dummy. He shows off a scrapbook of his former act, noting that they got kicked off of the show Match Game when he said "queef." Later, when Roger turns more psychotic than usual, Steve discovers that the scrapbook also contains clippings from where Dennis was blamed for a series of murders and sent away to the Danvers Asylum for the Criminally Insane. There, he finds Dennis who reassures him that it was indeed Roger who committed the murders, but in his rantings, also provides Steve with a clue as to the source of Roger's own psychosis.