Mr. Puff was Mrs. Puff's husband. He was caught and killed by some humans, and turned into a light cover for a lamp.
The only picture of him was in the episode, "Krusty Love," where he is shown getting turned on by a human hand pulling a switch. Mr. Krabs asked what happened to Mr. Puff, and SpongeBob replied that Mrs. Puff does not like to talk about what happened to him.
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