Dukey: He is the voice of reason in many episodes but often winds up joining in Johnny's crazy schemes and adventures (usually because of steak or Johnny tempting him into it). Despite his devotion to Johnny, Dukey frequently expresses deep disdain towards Johnny's less intelligent antics, believing their situation as master and pet reversed. He usually gets more stressed than his master in dangerous situations and often gets worried whenever there's a risky test for Johnny from his sisters.
Susan: Susannah Laura "Susan" Test is the twin sister of Mary Test and is the youngest of the two twins. She is one of the two tritagonists of the series and the older sister of Johnny Test.
Mary: Marrionette Louise "Mary" Test is one of the two tritagonists of Johnny Test. She is the oldest of the two twins.
