He is a wrinkly version of SpongeBob. He also has a larger forehead, two chins, and squiggly arms and legs.
Role in episode
When SpongeBob takes Old Man Patrick and his old friends to Bunny Buns, they all misbehave which causes him to get so over worked that he turns all wrinkly and old like Patrick. He then tries to walk until he declares that he is "losing steam" and steam shoots out of his pours. The steam turns Patrick back to his young self.
Back at Shady Shoals, Old Man SpongeBob is seen napping outside and his heart monitor suddenly stops beeping. Patrick assumes that he has passed away but SpongeBob, who is young again, jumps out of the blanket and tells Patrick "Gotcha!" and the two share a laugh.