Patty used to be an overweight, unattractive girl who was bullied by everyone. She used to go to high school with Francis. Francis was the only person who wasn't completely mean to her in high school, telling his friends that "She's not that fat." This made Patty like Francis and even fall in love with him. She kind of obsessed over him too, which he was annoyed by. Francis never actually let her know how he hated her because he wanted to show some decency for her. Francis would later be sent to military school, where he and Patty were completely apart from each other so that she would never see him again.
Sometime, shortly after Francis went away, Patty must have realized that she wasn't attractive and tried to better herself. She started to work out and get a lot of exercise and it really paid off because she lost a ton of weight and looked completely different than what she looked like before. Patty looked like a beautiful girl. She then got a job working as a babysitter.
Lois hired her to babysit Reese, Malcolm, and Dewey while she and Hal were away at a work convention and she didn't show up until two hours after they left. The boys were going to pull a bunch of horrible pranks on her, assuming that she'd be an ugly, disgusting old lady, but when she was shown to be a gorgeous, young teenager, they got rid of all their traps and welcomed her with a lot of love. The three of them turned their love for her into a competition against each other. Each one of them going against the other two, trying to impress her. However, Malcolm did nothing but alienate her with disturbing scientific facts about mad cow disease, and Reese did nothing but gross her out by belching while talking. Dewey worked his charm the best on her, making her think that he was cute and so she paid all of her attention to him. Malcolm and Reese both joined forces and went against Dewey, but everything they did only advantaged him.
Francis called the house while Patty was there and she started talking on the phone to him. Francis, oblivious to the fact that she changed her appearance entirely, was annoyed with the fact that he had to talk to her. She talked to him for over an hour and then started to lead him on. Francis finally let her have it at this point, telling her that he would never find her physically attractive, but maybe someday, someone else would. This really offended her and even left her in tears. Fortunately, Dewey was able to cheer her up, much to the dismay of Malcolm and Reese. In the end, Dewey, being the youngest, was allowed to sleep with her, and he also tricked Malcolm and Reese into getting locked outside.