He is actually Rigby's younger brother (despite him being of larger size and having a deeper voice than Rigby). Don admires Rigby and attempts to be like him; for example, Don claims that he stopped wearing clothes because Rigby started walking around naked. Despite this, Rigby resents Don because everyone seems to prefer Don's company over his own. While Rigby is poorly educated, usually penniless and hardly gets things done, Don is an accomplished accountant, drives a Corvette and has his own house.
Don gives everyone a shake-hug (hugging someone while patting them on the back or shoulder with one hand) which he calls "sugar." At first, Rigby hates him, but at the end of the episode "Don", Rigby finally gives Don some "sugar" and is on better terms with him. He is at least a year younger than Rigby and, despite being the younger brother, Don is the opposite of Rigby; Don is nonviolent and mature. He is still easily-relaxed and very nice to others, always offering sugar to possibly anyone, which made him more popular than Rigby. Don later admits that he did not want to overshadow Rigby, since he tried to be like Rigby as much as possible.
Currently Rigby and Don are reconciled and love each other like brothers and get along well as in the episode "Bank Shot" "The Thanksgiving Special", "Just Friends", and "Gymblonski". He was metioned in "Rigby in the sky with a Burrito", and "The Lunch Club".