A scene in HARRY POTTER AN THE KILL OF SNAPE reveals that when Roslalonde Lettuce didn't quite handle the car she was driving in the city of Portal High School, it crashed into a zoo. This, in turn, broke the koala exhibit and allowed the koala to escape. She was found by Scot and Snipper and immediately adopted by them, seeing as koalas are Snipper's favorite animals. As well as that, Snipper can communicate with koalas because he is "Austrian".
Once she's found, though, Scot and Snipper must rush to Portal High School. Scot decides to put Katty in his baseball bag, but during class discovers that this was a bad idea, since Katty was eating the (base)balls. Scot thus goes to the bathroom to take her out, but finds that he has entered the female bathroom.
Scot has to make a run so that the naked girls won't get him, and in the process loses Katty. She is found when she chews on Snipper, and reveals to him that Spy has tried to seduce Caroline against her will. She is also briefly seen in the senior graduation party, where she takes a male koala named Bund.
Lastly, Katty appears in the heat of the battle between Scot and his brothers. Snipper throws her at Wulf, and she begins eating Wulf's testicles. This helps in defeating Wulf, and Katty joins the group hug between the human protagonists of TEEN FORTRESS 2.