Calvin's Uncle Max appeared in a series of strips in 1988, visiting the family. Uncle Max is Calvin's father's "big brother". Like typical brothers, they often joke on each other. When Calvin (who first suspected he was a con man trying to swindle them) guesses that Max was in jail (to explain why he hasn't met Max), his mother is outraged, while his father seems to agree with Calvin, saying "with Max, that's not a bad guess". He seems to be a lot smarter and up to date than Mom and Dad, and is more in tune to Calvin's interests. He seems to have a good relationship with children and does not take Calvin's jokes literally, which is the main reason why Calvin and Hobbes like him. Max is not married and does not appear very wealthy. He seems like he has a very attentive job which may explain why he left so early. He does not think Mom and Dad do enough to raise and control Calvin. When Calvin snuck into Max's suitcase to try to find his present, Max, annoyed, asks, "Did your Mommy and Daddy raise you like this, or did they just untie you for this visit?"
Watterson had said Uncle Max was intended to be an expansionist character and include him in further strips such as where the family would go to Max's home to pay him a visit. He never appeared again after the week wherein he was featured in the strips, because Bill felt in the end that Max was not a great idea. A substantial reason for this was that it was strange for Max to be unable to refer to the parents with proper names, as Watterson never gave them any, and that the character never provided any new material for Calvin that he had hoped for. This may also explain why none of Calvin's other relatives appear in the strip.
It is unknown whether Uncle Max has a wife or children, but as far as the strip goes he doesn't. He somewhat resembles Groucho Marx.