- "I can't believe it. You can't be my twin sister."
- —Susan arguing with Mary at "Johnny to the Center of the Earth".
Susan Test is the twin sister of Mary Test and is the youngest of the two twins. She is one of the tritagonists of the series.
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 gets 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 is head over heels over Gil. She does things with Mary from watching him from satellite in their lab and trying to hang out with him. Also, despite the fact that Gil has been living next door to the girls for most of his life, he often forgets them, much to their dismay.
Susan is Johnny's sister and usually assists Johnny and Dukey with their creative inventions.
Susan loves her father, but despises his meatloaf and his constant punishments for doing experiments and testing them on Johnny.
Both have a good Mother-daughter bonding.
Sometimes Dukey really annoy both Mary and her but they he still has respect for both of them as his other owners.
Bling Bling Boy has a crush on Susan for a long time. However, she doesn't reciprocate the same feelings.
Susan has completely very long straight red hair. She 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 (albeit somewhat smaller than those of other characters). She wears a blue shirt with a star on it, a black skirt and pumps with her trademark star-shirt, and black Mary Jane shoes. She is always seen wearing a lab coat. She wears a star-shaped barrette in her hair. She sometimes wears a turquoise one-piece swimsuit; this is usually when sunbathing or at the beach. She also has suits for certain controlled experiments, 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. In Magic Johnny, she wears a blue shirt with a star on the chest, a blue french-painting hat, and her skirt. Her disco outfit is a turquoise tank top revealing her midriff, a turquoise hat, a black skirt with a pink belt around it, turquoise over-the-knee socks, and pink shoes. Her pajamas are deep-sky blue with white shining stars.
- Her counterpart in Leadbelly is named Simon West.
- The colors on Simon's shirt switch places with the colors on Susan's shirt.
- 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".
- Susan has a bigger temper than Mary, similar to her mother's.