In her first appearance, Cynthia mentions that her dad wanted a quiet life and 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 for socializing, but the party is butchered by Malcolm, fearing she would humiliate herself. In the end 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. It is also revealed that she has developed very large breasts for her age, something which she constantly hid from people out of embarassment, 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 in season 4 as a high school freshman along with her fellow Krelboynes. 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.