Roko-chan, formally named Hiroko Haruna, is Hamtaro's primary owner and a central human character. She is a 10-year-old girl who moves into a new house with her parents, her dog Brandy, and her hamster Hamtaro. Her daily life and activities frequently spark the hamsters' adventures. Family trips, like visits to the beach or her grandmother's sunflower field, become settings for Ham-Ham escapades. Her school life is sometimes depicted, showing interactions with classmates Kana, Travis, Kylie, and June, who own other hamsters in the group.

Roko-chan's relationships extend beyond Hamtaro. She helps her grandmother locate a lost friend, showcasing her supportive nature. Her birthday acts as a narrative catalyst in the movie "Hamu Hamu Land Daibōken," where Hamtaro and the Ham-Hams seek a magic sunflower seed after she becomes preoccupied with celebrations. On another occasion, she falls ill, prompting the Ham-Hams to rally for her recovery.

Musical themes associated with her include "Your Dream, Roko-chan," "Farewell, Roko-chan!," which underscore emotional moments, and the song "Ashitamo Asobou" (Let's Play Again Tomorrow). Her role remains consistent as a kind and responsible owner.