Haruka Hoshinogawa is a diligent, creative, and friendly fifth-grade elementary school student living in Sakura Town with her parents, Chu Hoshinogawa and Midori Hoshinogawa, and her grandfather, Ginzo Hoshinogawa. She leads the Sakura Town Protection Squad. A cherished childhood memory involves making a wish with her grandfather under the town's old cherry blossom tree, where he gifted her a treasured cherry blossom ribbon. When the tree began wilting, her grandfather entrusted her with managing his antique shop before leaving to explore the world. Soon after, the ribbon produced an egg that hatched into Ribbon, a Cocotama—a miniature deity born from beloved objects. Haruka accidentally discovered Ribbon, causing her to retreat into a Hiding Egg; this egg transformed into the Key of Wonders upon contact. The key identified Haruka as the Legendary Cocotama Contractor, destined to open "the door of happiness" and safeguard the world alongside Cocotamas. Her unique status allows her to see and interact with all Cocotamas without formal contracts, preventing them from disappearing.

Haruka has blonde hair tied in a side ponytail with a red bow, blue eyes, and typically wears a sleeveless hooded jacket over a white shirt, a blue skirt, white legwear, and pink footwear. While friendly towards schoolmates, she struggles academically. As the Legendary Contractor, she balances managing the antique shop with responsibilities towards Cocotamas like Ribbon, Pillow (born from a pillow), Chaco (born from a cup), Ruby, Marme, and Pantonio. She facilitates their integration into human spaces, establishing a Cocotama restaurant and clinic, while mediating conflicts involving mischievous Cocotamas like Tuxy and Dressy.

Her journey involves combating the Minus Power, an antagonistic force causing apathy and environmental decline linked to the cherry blossom tree's deterioration. In episode 55, she and the Cocotamas travel to Tsurukame Town to investigate Minus Power manifestations there. Her birthday coincides with the Tanabata festival on July 7th. The stakes escalate when the wilting cherry tree threatens to create a "Bad Future," a dystopia resulting from centuries of human disinterest. This is ultimately averted through combined efforts with Cocotamas from prior series. Her development culminates in the finale where Ribbon addresses the audience about cherishing experiences.