It is about a sixteen-year-old teenager who greatly overreacts to trivially upsetting situations and literally turns into a comically violent and destructive monster, usually leaving the family home in ruins.
In human form she is a very beautiful teenage female. She appears to be 16 and has a short blonde bob. She has a light blue shirt that somewhat hangs from her shoulders, pink pants and blue slip on shoes.
Before she changes into a monster her eyes glow, her voice changes, and she grows. In monster form she seems to be able to change into different forms. As one monster she looks like a green monster and her clothes are ragged, but she has also been seen as a fiery monster. However, after she explodes, she is fine and her clothes are unaffected while the rest of her family and nearby surrounding is completely ruined.
Katie doesn't seem to be aware of her exploding and turning into a monster after she has blown up, but she might just not comment on it.Copyright http://animaniacs.wikia.com/wiki/Katie_Ka-Boom