She has brown hair and pink bows on her pigtails. Karen wears a light-pink shirt and darker pink pants. She has a light-green elephant plush toy that she carries around all the time.
She becomes quickly offended, when Sōsuke didn't give in her demands.
In kindergarten Sōsuke tries to hide Ponyo. Karen goes to him. Then she asks him to play with her. But he refuse to do so. She shows her back and go away. Sōsuke uses the opportunity to look after Ponyo.