Haruka Shigure is a central character in the series, portrayed as a 15-year-old student at Miyasaka High School. She is easily recognized by her shoulder-length brown hair, which features distinctive purple tips, and her deep sky-blue eyes. Her typical attire consists of the standard school uniform, which includes a lavender pink skirt, a matching pink blouse, and a yellow ribbon. She is also a member of the school's tennis club.

As the childhood friend and classmate of the protagonist, Taito Kurogane, Haruka is defined by her sweet, caring, and affectionate nature. Her feelings for Taito go beyond simple friendship, as she has harbored a deep romantic love for him for a long time. This affection manifests in her daily actions, such as preparing elaborate lunches for him, gently waking him when he falls asleep in class, and wiping food from his face without any concern for what other students might think. She consistently prioritizes Taito's well-being, offering him both emotional and physical support, especially after he suffered a career-ending karate injury. However, her gentle demeanor is challenged by the arrival of Saitohimea, a vampire who shares a profound and intimate bond with Taito. Witnessing their connection triggers intense feelings of jealousy and insecurity in Haruka, causing her significant emotional distress as she questions her own romantic prospects.

Despite her ordinary appearance and personality, Haruka is intrinsically linked to the series' supernatural lore. She experiences unexplained dissociative episodes, during which she suffers from complete memory lapses and finds herself in locations with no recollection of how she arrived. In these moments, a completely different personality surfaces. This alternate self, sometimes referred to as the observer, is not human. It was reportedly created from the real Haruka for the purpose of watching over Taito. When this personality takes control, Haruka's appearance changes: her hair darkens and her eyes shift to a golden-yellow color. This observer personality acts as a spy, gathering intelligence on Saitohimea and Taito without their knowledge. The motives behind this surveillance remain somewhat obscure, but it is connected to factions interested in Himea's powers.

Her role in the story is heavily defined by her relationship with Taito, which serves as a key emotional anchor and a source of romantic tension. While she begins as the kind and supportive childhood friend, her life becomes intertwined with the supernatural prophecy at the heart of the narrative. It is eventually revealed that Haruka and Saitohimea are intrinsically connected, representing the two halves of a single, powerful entity known as Ramiel Lilith. This revelation positions Haruka as the human counterpart to Himea's supernatural existence, entangling her fate with the potential destruction of the world. Over the course of the story, Haruka's development involves the gradual unveiling of this dual nature, forcing her to confront the existence of her alternate personality and her hidden connection to the series' central conflict. Her abilities are tied to this other self, as she possesses no conscious supernatural powers. However, her observer personality is capable of independent movement and espionage, and her original personality has shown a latent, significant connection to the world's fate.