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