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 butchers the party, fearing she would humiliate herself. 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. 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 asked Reese 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.
Cynthia is a nerdy, quirky, and socially awkward girl. Needless to say, she fits right in with the Krelboyne clique. However, she is way more open and outgoing than the other Krelboynes. Instead of being closed off and aloof from the rest of the school, she puts herself out there and tries to make friends. However, she makes herself look really weird when she does so. However, she doesn't seem to notice or care. She is a very delightful, happy girl, who only wants to make friends and doesn't worry too much about her appearance in front of others. She tries a bit too hard to make friends, however, as she comes across as obsessive and stalkerish at times.
In the episode Cynthia's Back, Cynthia's absence was explained, by saying that she had done a semester in Europe. Over the semester, she went through puberty and her personality went through a sudden and abrupt change. She started to be a lot more emotional and had negative mood swings, acting very cruel and disrespectful, and was no longer the silly, peppy girl that she used to be. She had started to act very cold and distant to the other Krelboynes, and if they tried to interact with her, she would abuse them. This behavior was toned down at the end of the episode, but it wasn't entirely forgotten.
In the episode Humilithon, Cynthia mainly exhibited the same character traits she did, before she went through puberty, still being the socially awkward, embarrassing, nerdy girl she used to be. However, her mentality was a lot more adult, as she helped out Malcolm, by telling the entire school that they had sex with each other, in order to boost his popularity. So, although she humiliates herself in public, she's aware of what she's doing to be awkward and she knows how to be popular, but refuses to do so, because she likes herself for who she is, regardless of whether she's popular or not.
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 developped a crush on her. They almost kissed several times, but due to Malcolm clumsiness, 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 highschool, 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 ahold 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 backbrace with a refridgerator 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 mesurement, which didn't work at all.