Rosy is first introduced to the audience as a grandmother looking for Balto's statue in New York Central Park with her granddaughter. In the film she's a young girl who has an interest in dog sledding. Her parents commissioned Mr. Johansson to build her a sled and was surprised with it during a sled dog race. She then hitched up Jenna, a purebred husky, and went for a ride.
Within a few days, along with many other kids, Rosy had become ill and was taken to see Dr. Welch. Jenna watched Rosy being examined from a window, trying to understand what's wrong with her. Balto shows up to invite her out, but he sees that Jenna is upset. When Jenna explains that Rosy is ill, Balto takes Jenna underneath the hospital (to listen in) where they find out that Rosy has Diphtheria. Having no antitoxin to give Rosy, she was placed into the hospital where she would rest until a new batch of antitoxin was brought in. When Balto did bring the medicine, he was allowed into the hospital where Rosy told Balto "I would be lost without you".