Hiyori Kazane is a second-year student at the same school as Tomoki Sakurai and the central figure of Heaven's Lost Property the Movie: The Angeloid of Clockwork. Her existence carries a profound mystery, as she lives a dual life. While she appears to be an ordinary, hardworking girl on Earth, her true form is that of an angel, first seen by Eishiro Sugata sleeping in a chamber within the ethereal realm known as Synapse. This chamber connects to her body on Earth, which functions as an avatar, allowing her to interact with the world while her real self remains dormant. Her life on Earth is marked by quiet responsibility; after the death of her parents two years prior to the main story, she took over the care of the family farm and looks after her younger siblings.

Hiyori has a pure, humble, and deeply caring personality. She is somewhat shy and easily embarrassed, often blushing in awkward or heartfelt situations. Unlike many other characters who are quick to point out Tomoki's faults, Hiyori is consistently kind and gentle toward him, a quality that becomes a source of discomfort for Sohara, Nymph, and Astraea. Her primary motivation is her sincere romantic feelings for Tomoki, which she has harbored for some time, even observing him daily from her family's garden along his route to school. To get closer to him, she musters the courage to join the New World Discovery Club. Although Sugata is initially hesitant due to his knowledge of her true nature, he allows her to join, partly because her membership brings additional funding to the club. Tomoki even subjects her to a series of embarrassing, fake initiation tests intended to make her quit, but she completes them all with determination, further solidifying her place among the group.

Her role in the story is defined by tragedy and transformation. After successfully confessing her feelings to Tomoki, a shocking event occurs when a careless truck driver runs her over. This accident causes her earthly avatar to disappear, and her existence is erased from the memories of almost everyone, including her own school records and photographs, leaving only Tomoki and the angeloids Ikaros and Nymph aware of her. In the wake of this, she is later converted into a Type Zeta Angeloid by the Master of Synapse, who uses her as a weapon to destroy the city. As a transformed Angeloid, she wields a staff called Demeter. In the manga, this staff grants her the power to manipulate the weather, including rain, lightning, and tornados, as well as the ability to alter air pressure to incapacitate people. In the film adaptation, her abilities are depicted as space-time manipulation, allowing her to foresee and nullify attacks or transport objects through dimensions. Regardless of the medium, she also possesses powerful hacking capabilities that can even counter-hack Nymph, though her physical durability remains low due to her originally human nature.

Hiyori's key relationships are central to the plot. Her love for Tomoki is the driving force behind her actions, from joining the club to ultimately breaking free from her master's control. When Tomoki returns her bell-adorned hair ornament and speaks to her, it reawakens her memories and free will, allowing her to resist her programming. Her sacrifice is a pivotal moment; she self-destructs after using her own abilities to dispel Ikaros's protective Aegis shield, sharing a farewell kiss with Tomoki before her apparent destruction. Her development thus traces a poignant arc from a timid, infatuated schoolgirl to a tragic figure who is manipulated and weaponized, ultimately achieving a bittersweet liberation through her own sacrifice. While she quits the New World Discovery Club after being freed, she continues to harbor feelings for Tomoki and later plays a crucial role in helping the Angeloid Chaos grasp emotions, showing that her innate kindness persists even after her profound ordeal.