Frankie Foster is the pretty, capable, easygoing granddaughter of Madame Foster. According to her driver's license, Frankie is 5'10" tall, weighs 127 pounds, and has green eyes; she is 22 years old.
Frankie has apparently lived at Foster's nearly her entire life, having moved there in her early childhood. It is unclear what happened to her parents (even though never confirmed, they may have died); she seems to have lived a good life with her eccentric but lovable Grandmother.
As she grew into her teenage years Frankie, who had spent her life in the company of Imaginary Friends, soon took on the role of taking care of pretty much everything at Foster's. She was placed in charge of cooking, cleaning the house, doing the laundry, running fund raisers, occasionally working as a handyman, driving the house residents around in the multicolored bus, and otherwise taking care of her grandmother's foster friends in most ways. Mac once referred to her as "the estate manager." Yet despite the fact that Frankie basically runs the house, she is still required to answer to Mr. Herriman.
She occasionally shows sign of stress as a result of her many duties, though its primary source seems to be Herriman's constant over-enforcing of the house rules and that he forever expects her to work harder despite her already full workload.
Still, despite all her work, she does manage to keep a social life and is even allowed to go on the occasional date (assuming Mr. Herriman doesn't keep her working late with more chores). She is also often swayed by Bloo's "get rich quick schemes" and has proven to be an efficient ally in promoting Bloo's agendas when she feels she can get a good profit or an outcome out of it.
She is a fan of punk rock, She is also proficient in web-design, creating and maintaining the Foster's web-page.