Haiji is a small, doll-sized guardian spirit who serves as a loyal familiar to the young vampire Hazuki in the anime MoonPhase. Her origins are tied to the mysterious and magical elements of the series, as she was originally created by a woman named Shizuru, who is Hazuki's mother. In her earliest form, Haiji served Shizuru while taking the shape of a gray cat. Her name itself is derived from her feline past, coming from the nickname Haijiro, which was shortened to Haiji.

In her default appearance, Haiji looks like a small, flying figure with cat-like ears, and she possesses the notable ability to transform into a black cat for the purpose of camouflage and stealth. She is frequently seen floating and flying through the air, adding to her whimsical and supernatural presence. Despite her small stature, she is a capable helper around the house and acts as a brave protector for her mistress.

Personality-wise, Haiji is characterized by her sharp tongue and a mischievous, outspoken nature. She is not simply a quiet pet or servant; she actively participates in the daily banter of the household. She often takes on the role of an arbiter or referee, showing great amusement when acting as a judge during the frequent petty arguments and rivalries between her mistress Hazuki and another character named Hikaru. She is also not above scolding the main protagonist, Kouhei, for his actions, adding a layer of comedic relief to the series. Her dialogue often reflects a dry, matter-of-fact attitude, as seen when she wryly questions the state of the household's kitchen organization.

Within the story's framework, Haiji serves as a constant companion to Hazuki, following her on various misadventures, such as traveling to Kyoto to seek mystical assistance or even declaring a comedic war on garden crows that steal her mistress's lunch. Her relationship with Kouhei is a frequent source of humor; while she serves his household, she often maintains a critical and teasing demeanor towards him, referring to him as "master" with a hint of sarcasm. Her loyalty, first to Shizuru and later to Hazuki, is the central pillar of her character. While she is a product of magic and her own memories of her earliest origins are unclear, her devotion to the family line she serves remains absolute.