Kari seems to be very optimistic, and cheery. Even when Jack-Jack is causing mayhem, and she is desperately calling Helen Parr, she still has an optimistic tone in her voice.
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).
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.