Mr. Marbles is a ventriloquist dummy that Kramer owned. In “The Chicken Roaster”, Jerry and Kramer switch apartments. After the first night, Jerry suggests things that Kramer should get rid of, one of which is Mr. Marbles. Jerry fears that Mr. Marbles will kill him in the night, and Kramer assures Jerry he is harmless. That night, Jerry awakens and sees a silhouette of a doll run across the room laughing, terrifying Jerry.
Mr. Marbles is an obvious reference to Chucky, a doll in the Child’s Play movie series, which is possessed by a serial killer.