Susan Test is the twin sister of Mary Test and is the younger of the twins.
Susan is a super genius scientist. She is the younger twin of her sister Mary. Susan has been shown to be less empathetic and sympathetic than her sister. Like her sister, Susan harbors a deep love and obsession for Gil. Bling-Bling Boy (A.K.A Eugene) has a huge crush on Susan but whenever he tries to get her to fall in love with him it always fails, mainly because Johnny foils him or Susan turns him down (Susan always rejects Bling Bling Boy's advances, as she finds him repulsive). Like her sister, she attends school at the Porkbelly Institute of Technology. Susan is prone to believing in the more scientific and logical explanations for things rather than the illogical and magical explanations. Her sister is more open-minded, and Mary is frequently proven correct. Susan seems to be less level-headed than her twin Mary, and tends to be less conservative, as she speaks her mind much more frequently.
She has completely straight red hair and wears square glasses. Without her glasses she has very poor vision and her eyes show strain. In Downhill Johnny, her eyes are big blue dots, whereas in later episodes she has normal eyes (albiet somewhat smaller than those of other characters). She wears a blue shirt with a star on it and wears a star-shaped barrette in her hair. She is always seen wearing a lab coat, black skirt and pumps with her trademark star-shirt, and black Mary Jane shoes. She sometimes wears a turquoise one-piece swimsuit; this is usually when sunbathing or at the beach. She also has suits for certain controled expirements, such as a hi-tech diving suit and a skin tight yellow hazmat suit. Both have her trademark star on the side. Sometimes, she will don a pair of blue latex gloves and a surgical mask when handling bio hazardous materials, namely during surgeries.
- Her counterpart in Leadbelly is named Simon West.
- Susan and Mary's catchphrase is "We're such geniuses".
- She and Mary are extremely ticklish, as was revealed in the episode "Johnny vs. The Tickler".
- She and Mary are identical twins, but people can tell them apart by their eye colors and hairstyles (Susan's eyes are ocean blue, while Mary's are more of a teal/jade. Also, Susan has straight hair, while Mary has curly hair).
- Susan has a bigger temper than Mary, similar to her mother's.