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. Later when the two are back in their respective apartments, Jerry once again experiences Mr. Marbles coming into his room. A deleted scene from the episode also showed the silhouette of Mr. Marbles with a knife in hand approaching Jerry as he lies in bed.
In a deleted scene from "The Andrea Doria", Kramer puts Mr. Marbles into storage, with Jerry surmising that Kramer too was unnerved by the dummy.