Kari McKeen is a 13-year-old babysitter. She is a friend of Violet, who hires her to watch Jack-Jack when Violet and Dash decide to tag along with their mother. The enthusiastic Kari speaks through "a mouthful of orthodontia," and considers herself an expert on how to deal with babies, such as playing Mozart to increase their intelligence. Helen is not entirely confident about a teenage girl watching over her infant son, as she prefers a full-grown adult to do that job instead (which ultimately does happen, but with far worse results than Kari's oversight of Jack-Jack).

Despite her training and optimism, Kari is unprepared to deal with Jack-Jack's emerging powers, and leaves increasingly frantic phone messages with Helen, before Syndrome steps in and introduces himself as a more experienced "replacement sitter". Kari is clearly unaware of Syndrome's checkered criminal past; only asking him why he wears a big "S", which Syndrome calmly says it stands for "Sitter". Kari's ordeal remains off-screen in the film, but serves as the focus of the short film Jack-Jack Attack. She is last seen being interrogated by National Supers Agency government agent Rick Dicker, to whom she shouts hysterically "The baby was exploding!" She then wishes to forget the whole ordeal, to which Dicker promises that she will as he activates a device to erase her memory of the incident.

Her surname is only mentioned in the screenplay and in Jack-Jack Attack.