Kari is a fourteen year-old babysitter. She is the best friend of Violet, who hires her to watch Jack-Jack when his siblings 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; preferring a full-grown adult 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 Rick Dicker; and claims she would like to forget the whole ordeal, to which Dicker is more than happy to grant her request.
Her surname is only mentioned in the screenplay and in Jack-Jack Attack.
Kari does not appear at the end of the movie or in any of the comic book stories, leaving her fate unknown (however, it is most likely that she and Violet remain friends after Dicker erases her memory, seeing as how there wouldn't be any issues on Violet's secret identity).
It is shown through concept art that Kari (and even her parents) was supposed to return for the sequel, but she and her family were scrapped for unknown reasons (presumably because she served little to no purpose in the story). However, Edna Mode mentions to Bob that music from Mozart makes Jack-Jack's powers ensue, which is a reference to Kari's famous line, "Mozart makes babies smarter".
The deleted scenes reveal that while Kari has returned to her usual quirky self, her parents have stated that she has become "twitchy" and is completely oblivious to certain things around her. She also has a habit of giggling for no reason. Despite being relatively happy, Kari's parents have noticed that something is different about her and are upset about it.