In her first appearance ("Krelboyne Girl"), Cynthia mentions that her dad wanted a more quiet life, so he moved from Manhattan to Malcolm's hometown. Being the new kid, she constantly tries to fit in with her peers, even going so far as to plan a party specifically for socializing. Unfortunately, Malcolm tries to stop her from throwing the party, fearing that she would only be disappointed with no one attending if she does it. However, his efforts only humiliate Cynthia and she cancels the party. After being confronted by Stevie, Dabney and Lloyd who reveals that everyone in the school hates him and not Cynthia, Malcolm arrives at her house to apologize for his earlier behavior. He thought he was saving her from the embarrassment of having no one attend, but was just making things worse for himself. Malcolm mentions that he realized that Cynthia was right about him being too uptight over his own ego. In the end, however, the party is a success, and Cynthia makes new friends.
Her prolonged absence from season 3 is explained that she was on a vacation in Europe with her father in the episode Cynthia's Back. It is also revealed that she has developed very large breasts for her age, something which she constantly hid from people out of embarrassment, and was pushy and mean as a result. She wore large sweaters to hide them, but they are accidentally revealed to Malcolm and he promises to keep that a secret. The next study buddy date, Cynthia briefly returns to her normal self and is laughing at the jokes he's telling her about the class. Reese then ruins the moment by insulting her and Malcolm is offended Despite promising to keep them a secret, he accidentally told Reese, who started a relationship with her only because he wanted to see them. Malcolm did everything he could to prove Reese's true intentions, but she thought he was just jealous. However, when all three of them were together, Malcolm finally outsmarted Reese by asking him what her name was. Unable to remember, Reese grabbed her breasts in desperation and she beat him out of anger for manipulating her.
She made her last appearance along with Kevin and Lloyd in the episode Humilithon. When Malcolm was about to make a huge mistake by destroying Lois' car, she desperately attempted to stop him and did so by claiming they had sex ultimately destroying her reputation in the process, after this she is never seen again.
When Cynthia first met Malcolm in Krelboyne Girl, she quickly developed a crush on him. She was very open and friendly to him, and she instantly considered him to be her best friend. Malcolm, personally didn't like Cynthia at first, but that didn't let Cynthia stop following him around. She talked about her weird stories and always stalked him around the school, just trying to fit in with someone else at the school. Malcolm also developed a crush on her. They almost kissed several times, but due to Malcolm’s clumsiness and other awkward circumstances, that didn't happen. Later in the series, Cynthia became more of a friend to Malcolm, than she was a stalker. When she and Malcolm went to high school, in Humilithon, Cynthia noticed that Malcolm was becoming the laughing stock of the school and tried to help him. She tricked the entire school into thinking that she was sexually active with Malcolm, which raised his popularity again.
Cynthia does not like Reese, because he's annoying, disrespectful, stupid, and gross. When she first met Reese in Krelboyne Girl, she arm wrestled with him and instantly won, painfully twisting his arm and scaring him away. Cynthia is a lot stronger than Reese, because this isn't the only time she's physically overpowered him. In the episode Cynthia's Back, Cynthia went through puberty and grew boobs, and when Reese learned about this, she fell in love with him. He then started to be genuinely nice to her and tried to start a romantic relationship with her, solely for the purpose of getting a hold of her boobs. Malcolm warns her before it's too late and she ferociously attacks Reese.
When Cynthia returned in Cynthia's Back, Dabney was a major target for her bullying and abuse. It was mentioned that she stuck her homework to his back brace with a refrigerator magnet as a joke. Dabney held a grudge against Cynthia, and tried to make a comeback, by outsmarting her with his knowledge on units of measurement, which didn't work at all.