When Mark Rowe arrives on the island of Baladora with Carolyn Hawlings to meet with his Aunt Benna, instead of being greeted by her, he is instead greeted by a man named Dr. Richard Hawlings and his twelve-year-old daughter, Kareen. They inform Mark that his aunt has been missing for weeks, and that they need the jungle magic that was bestowed on Mark to find her. It is revealed that Mark was bestowed the magic when he was four by Aunt Benna, however Mark has no idea how to call upon it. Mark walks off, only to find a room filled with shrunken heads and Aunt Benna's journal that says that Dr. Hawlings and Carolyn can't be trusted. He ends up running into Kareen, who claims she doesn't know anything about her dad being evil. Mark believes her, but goes off into the jungle on his own to find his Aunt Benna.
After a series of harrowing incidents in the jungle, Mark is eventually rescued by Kareen. The two find Aunt Benna, who is horrified to see Mark with Kareen, as it turns out that she was working with her father all along. And due to him saying the magic words "Kah-lee-ah" in front of her, she now knows. However, they can't get the magic to work. They threaten to shrink Mark and Aunt Benna's heads.
Mark attempts to use his Shrunken Head on them, but it gets knocked into a pile of heads. Mark dives into the pile, finds the correct head (thanks to it having a white scratch on its ear from Mark's little sister Jessica) and says the magic words, causing Dr. Hawlings, Carolyn and Kareen to shrink to the size of mice and run off into the jungle, never to be seen again.
In the television adaptation, the villains are still shrunk, but Mark manages to catch them and keep them in a lizard tank in his room where he takes care of them and the spell will be removed when Aunt Benna comes, but only until his sixteenth birthday.
Kareen is rather bratty and snotty, especially towards her father. She acts kindly to Mark until that's eventually revealed to be an act to let his guard down and lead her to Aunt Benna.
Kareen is described as having straight blonde hair and blue eyes. She wears a white t-shirt and white tennis shorts.