Uncle Max appears to be somewhat younger than Calvin's father, with more hair and a vigorous attitude. He has beady eyes, stiff hair, and a bushy moustache, which make him somewhat resemble comedian and film actor Groucho Marx. His attitude toward Calvin's rambunctious behavior was more laid-back than Calvin's parents', and he appeared far less rigid in his lifestyle, being single and childless. In fact, Calvin's father thought it likely that Max had been unable to visit as he was doing jail time.
Despite his initial reservations (he anticipated that Max might be an impostor and con man), Calvin quickly warmed up to Max, who cared to share his fantasies by pretending that Hobbes was alive and fearsome. As a result, Calvin was considerably saddened when his uncle left and even tried to go with him.
Behind the Scenes
Bill Watterson explained in The Calvin and Hobbes Tenth Anniversary Book that Uncle Max was intended to be an expansionist recurring character. However, his role was cut short because Watterson felt in the end that Max's inclusion was not a good idea. A substantial reason for this was that it was off-putting for Max to be unable to refer to the intentionally unnamed parents with proper names. Max created few jokes, and was hard to use, since Calvin's parents are unnamed. Also, the arc failed to create new material according to Watterson. The same reasoning explains why none of Calvin's other relatives appear in the strip. They too would be hard to fit in storylines with Calvin, Hobbes, and Calvin's parents.