Eileen A. Jacobson is Jason's friend and former girlfriend, though he denies this. She is extremely smart, with auburn-red hair and freckles. She is said to be ten, like Jason. It`s hinted that she also has a crush on Jason. She went to the same science summer camp that Jason and Marcus went to. Eileen and her friend, Phoebe Wu, had a prank war with Jason and Marcus at camp. The girls kicked their butts every time while the boys' pranks backfired on them. At the end of camp, the boys after an attempt to make them sign a peace treaty joined their friendship club. Eileen enjoys making Jason uncomfortable for laughs. She often hints that they have an obvious relationship. There was a series where she accidentally got a Valentine's day card from Jason, originally intended for Andy. Predictably, of course, chaos ensued. In the end, the entire school thinks they like each other and Jason admits to himself that he likes girls now.

Eileen is friends with Phoebe Wu and Marcus Jones. She and Phoebe became best friends at Camp Bohrmore, the summer camp mentioned previously. Marcus is friendly towards Eileen, except when Jason influences him not to be. Eileen and Phoebe have only appeared 4 times together. 2 in Sunday strips and 2 times in REALLY long story arcs. The first one was when they were at summer camp and the second was when Pheobe visited them.