Guni is a small, green, female winged creature who serves as a companion and assistant to Shido, the protagonist of Nightwalker: The Midnight Detective. Often described as a fairy or a demon, her appearance is somewhat reminiscent of a tiny gargoyle, and she is always depicted naked, though not in a graphic manner. Her diminutive size allows her to frequently travel while hidden within Shido's voluminous hair, making her an unobtrusive but ever-present figure at his side.

Guni's personality is defined by her sharp tongue and sarcastic demeanor. She is known for making biting, critical remarks, and she often irritates the other characters, particularly Riho, whose naivete she frequently mocks. Despite this outwardly prickly and precipitous nature, her behavior masks a genuine sense of care and loyalty. She has a definite soft spot for Shido, and her primary motivation is to protect him and offer sound counsel. In this way, she often functions as the voice of reason or Shido’s conscience, providing worthy advice even when it is delivered with a sarcastic edge.

Within the group, Guni acts as a constant observer and commentator on events. Her role is not primarily that of a fighter but rather a confidante and an advisor who helps Shido navigate the supernatural cases he investigates. Her relationship with Shido is the most significant; she is a trusted ally who has accompanied him for a considerable time, and her concern for his well-being is genuine. She is openly critical of Riho's inexperience and idealistic tendencies, creating a dynamic of friction and exasperation. Her relationship with Yayoi, the other main female member of the team, is less prominently featured but exists within the framework of their shared mission to combat the Nightbreed.

Guni shows little traditional character development, remaining a consistent source of sarcastic wit and loyalty throughout the series. However, her importance to Shido is powerfully illustrated in a pivotal nightmare sequence where a dark version of Riho kills her. This event, though a hallucination, serves as a deep psychological blow to Shido, underscoring how integral Guni is to his sense of normalcy and his found family. In terms of abilities, Guni does not possess significant combat powers. Her primary notable abilities are her power of flight and her capacity to hide in small spaces, most notably Shido's hair. Her true value lies in her intellectual and emotional support, offering her sharp observations and protective instincts in the face of danger.