Hiroko Haruna, often called by the nickname Roko-chan, is the ten-year-old owner of the golden hamster Hamtaro. She lives in a house with her parents and the family dog, Brandy. After moving to a new city, she initially felt shy but soon befriended a neighbor named Kana Iwata, as they bonded over their shared love for hamsters. This friendship helped her become more comfortable and connect with other classmates and neighbors. Hiroko is a student, presumably in the fifth grade, though she enjoys school, she consistently struggles academically and often receives poor grades. She also has a well-documented crush on a popular boy in her class named Travis Kimura, whose attention is also sought by other girls.

In terms of personality, Hiroko is described as having a kind heart and a fun-loving nature. She cares for Hamtaro with great devotion, and the two share an exceptionally strong and mutually affectionate bond. Her love for her pet is a central aspect of her character, and she is often seen caring for him or spending time with him at the end of the day, sometimes writing in her diary while he eats a snack. Despite her generally kind disposition, she can sometimes be impulsive, such as when she pouts dramatically or even runs away from home after her parents refuse to buy her a CD player.

Hiroko’s primary role in the narrative is as the human connection to the world of the Ham-Hams. Her daily life, problems, and feelings often serve as the catalyst for the hamsters' adventures. For instance, when she becomes preoccupied with her birthday celebrations, the Ham-Hams set out on a quest to find a special gift for her, and when she falls ill, they rally together to help her recover. Similarly, news of a family trip can spark a new adventure for Hamtaro and his friends. Her strong bond with Hamtaro is so deep that they sometimes share dreams, and he often becomes distressed if he feels ignored by her. Her relationship with her dog Brandy is another part of her home life, though Brandy typically sleeps and is largely unconcerned with the hamsters' activities.

Throughout the series, Hiroko shows some character development, particularly regarding her feelings for Travis. In a significant episode, after learning he plans to study in England, she is initially distraught but ultimately runs to meet him. They promise to remain friends despite the distance, and he later sends her a letter, leading her to work on a reply with help from her friends. Hamtaro even imagines a future where the two eventually marry. This ongoing narrative thread shows her maturing feelings and her determination to maintain a connection with someone she cares about. While not an ability in the traditional sense, one of her notable traits is her subtle, perhaps unintentional, communication with animals. She can talk to Brandy, and Hamtaro seems to understand her words and feelings, even if she is not aware of his adventures when he leaves his cage each day. Her daily routine concludes with her speaking to Hamtaro about the events of her day, which reinforces her role as his beloved owner and confidante.