The Test Twins are Johnny's 13-year old genius identical twin sisters, who frequently use him as a lab rat for various inventions, most of which to impress their neighbor, Gil. . Though they generally refuse to help Johnny in his antics, they generally end up doing so anyway due to Johnny blackmailing or manipulating them, or in exchange for Johnny allowing them to use him as a guinea pig. Their hard-headed demeanor makes them gullible, and they have been tricked by Johnny on various occasions. Both twins wear traditional lab coats and harbor a deep love and obsession for the Test's next door neighbor, Gil, although their attempts to attract his attention always end in failure. Both girls attend school at the Porkbelly (formally Mega) Institute of Technology.
Susan Test: She has blue eyes and wears a black skirt, knee-high socks and a pair of mary janes, and a blue shirt depicting a star. She has straight red hair, held with a star-shaped clip, wears square eyeglasses. She tends to be more jaded and cynical than Mary, which often leads to her downfall. Susan's emotions are more extreme than her twin's, and she tends to be more impulsive. Eugene/Bling-Bling expresses fond interest in Susan, often causing her to be used as a "bargaining chip" in his negotiations.
Mary Test: She has green eyes and often wears baggy blue jeans and sneakers with her trademark yellow moon-shirt. She has curly red hair, held with a crescent moon clip, and always wears crescent shaped glasses. She was only seen with her glasses off once and they were half-off. Mary has been shown to be more compassionate than Susan. She is prone to believing less in science and more in the realm of science fiction, which often proves to be correct, despite the scorn received from her twin. Mary seems to be more level-headed and conservative than Susan, and speaks her mind much less often, she also cares about Johnny more openly than Susan.
The twin usually work for the geniuses to help the maintage work out but still teasing and using Johnny as their test subject.