From Homestar Runner Wiki
Dijjery Doo, also spelled Dijjery-doo and Dijjery Do, is a brown "Cheat-like prankster" with two long tusks, which garble his speech somewhat. His left tooth has a habit of falling out when he talks. First seen in the Museum sketchbook, all of his appearances seem to be based on the sketch in the lower right, without stripes.
Dijjery Doo was originally abandoned by The Brothers Chaps in the design phase for being too "Cheat-like" and "walrus-like". He was not seen outside of the sketchbook until he made an appearance in a @StrongBadActual tweet in Mario Paint form. He was brought to life not long after in Homestar Runner Goes For the Gold, in which he is seen not only in Mario Paint, but also in Storybook and modern designs. At the end, it is revealed that he survived in a crate in Homestar Runner's shed for twenty years, though The Cheat prefers to keep him locked up.
In his official debut in Homestar Runner Goes For the Gold, he seems to be Strong Bad's lackey, although an incompetent one, as his attempt to sabotage Strong Sad fails disastrously. The Cheat seems to harbor a deep resentment towards Dijjery Doo, since he replaces The Cheat in the story. His name comes from the didgeridoo, a traditional Australian Aboriginal instrument.