He is beige and has a large gray beard. He wears a green sweater and blue pants, and uses a mobility scooter to move around.
When GrandPat was younger, the lactose intolerant movement cause Bikini Bottom to ban Ice Cream, much to the dismay of GrandPat and many of the citizens. During this era of prohibition, GrandPat went to Sweet-Easies where Ice Cream was sold and purchased in secret. One day, GrandPat attended a Sweet-Easy that was discovered by the police leading to GrandPat's arrest. While in prison GrandPat developed a taste for prunes since it was the only source of food the prison served. Eventually it became legal to buy and sell ice cream again.
In 1926, the FlimFlam Brothers borrowed $10 from GrandPat and skipped town. In the present day, he finds the two in his house and chases after them since they owe him restitution.