Betty Quinlan is Jimmy's first love interest and one of his best friends. Jimmy frequently tries to get her attention with his inventions and flirts with her, and Betty flirts back to Jimmy, seemingly charmed by him, much to Cindy's jealously. Betty attends Jimmy's school, but does not take any classes with him or his friends, and seems to be a few years older than him.
Betty's first appearance was in the episode Party at Neutron's, where she was one of the kids who attended Jimmy's party. She dances with Jimmy, and gives him a kiss on the cheek when she leaves his house.
Her second appearance was in Out Darn Spotlight, as the main protagonist. She was shown to be a talented actress, as she was given the role of Lady Macbeth in their school play "Macbeth in Space." Nick originally starred opposite Betty, playing Macbeth, but Jimmy was forced to take over the role after Nick was injured. In this episode, Betty was Jimmy's motive for wanting to be in the play in the first place, and she seems to know this, and offers him kind encouragement. Towards the end of the episode, Betty kisses Jimmy on the lips, because they had not gotten to do so on stage.
Betty's third appearance was in the episode One of Us, where she was hypnotized by Grandma Tater's The Happy Show Show, along with many other Retroville residents. When Jimmy saw Betty as a zombie, he ran away.
Betty's fourth appearance was in Vanishing Act, in which Jimmy attempts to perform a magic show in order to impress her. However, as a result of Cindy's jealous actions, the magic show goes awry, and they end up stuck in another dimension. Jimmy and Betty continue to flirt, which angers Cindy to the point of furiously attacking them over it and blames the whole thing on both Betty and Jimmy (which Jimmy calls her out on). Betty finally gets fed up with Cindy's nasty attitude and calmly pulls her aside explaining she knows everything and is not interested in Jimmy that way. She tells Cindy to stay out of her face, being sick of Cindy always being mean to her and that Jimmy is all hers. This implies that while Betty is flattered by Jimmy's affections toward her, she knows he belongs with Cindy and is fully supportive of it, but it's hard to show it because Cindy is always mean to her.
- Betty and Cindy's relationship is similar to the relationship between Betty and Veronica from the Archie franchise. However, Betty looks more like Veronica and Cindy looks more like Betty.
- During the cancelled season four, there would have been an episode called "Three's a Crowd", where Cindy, Libby and Betty would've gotten trapped in Jimmy's lab and have to work together to get out. In this episode, Cindy would've finally come clean about her resentment towards Betty, they would've opened up to each other and at the end, the girls would've become friends.
- Betty is implied to be slightly older than Jimmy and his friends, since she's not in any classes with them and is taller.
- It is revealed in The League of Villains that Jimmy still likes her, since he was shown to still have many pictures of her in a drawer in his lab, which Cindy made him get rid of upon discovering them.
- Sometimes, it is hinted that Cindy might be secretly fond of Betty.
- Besides the episodes in which she is a main character, she is also a background character in the show.
- "TIME OUT!! Cindy, we need to have a little chat... Look, I know all about the whole 'you and Neutron' thing...Just stay out of my face. He's all yours." -Vanishing Act
- (In Jimmy's daydream) "Oh, James..." -Operation: Rescue Jet Fusion
- "Kiss me, my king of the stars!" -Out, Darn Spotlight
- "Great party, Neutron!" -Party at Neutron's
- "Sure, Jimmy! I love magic!" -Vanishing Act