Kari is a thirteen year-old babysitter. She is a 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).
Kari ultimately goes over a crazy nightmare on her babysitting job do to Jack-Jack's powers suddenly manifesting. When Syndrome knocks on the door, she left Jack-Jack with him believing to be her replacement.
However, at the end of Jack-Jack Attack, her memory is wiped by Rick Dicker, after interrogating her of the events that happened while she was babysitting.