Kobeni Yonomori is a sixteen-year-old high school student who initially appears ordinary but handles all cooking and household chores for her family. She has shoulder-length salmon pink hair usually styled in two pigtails, violet eyes, and slightly pale skin, bearing a scar from a childhood accident. Her figure draws commentary for its large bust and "child-bearing hips," traits causing her self-consciousness and unwanted attention, amplified by being the younger sister of the school's student council president.

Her background revolves around childhood visits to the Mitsumine household. While playing in the woods with Hakuya Mitsumine, she fell from a cliff. Hakuya saved her by sharing half of his supernatural Inugami powers, sustaining severe injuries that scarred his right eye. This act erased Kobeni's memories of Hakuya and the Mitsumine family while granting her an extended lifespan. A side effect causes occasional fevers and a constitution weakened by the incompatible energy. On her sixteenth birthday, Hakuya and his younger sister Mashiro arrived at her home, enforcing an arranged marriage pact orchestrated by her late grandfather, forcing cohabitation that renewed their relationship. Manga revelations later show that Kobeni herself, at age five, originally proposed the marriage to Hakuya before the accident-induced amnesia, a fact Hakuya concealed until the series' end.

Personality-wise, Kobeni is generally sweet, optimistic, and responsible, though prone to scatterbrained moments. She copes with stress through shopping, like fleeing the house after learning of her engagement. Despite external pressures—including her sister's overprotectiveness and societal expectations—she remains resilient, adapting to unforeseen challenges. Her initial discomfort with the arranged marriage evolves into genuine affection for Hakuya, shown through blushing reactions to his compliments and jealousy when other girls pursue him, culminating in formal dating and eventual marriage.

Key relationships define her experiences:
- With Hakuya, she progresses from awkward distance to mutual trust and romantic commitment, often discerning his emotions despite his typically expressionless demeanor.
- Her older sister Benio exhibits extreme protectiveness rooted in guilt over the childhood accident. Kobeni respects Benio but finds her overbearing, frequently attempting to temper her sister's obsessive behavior.
- With Mashiro, her future sister-in-law, their dynamic begins blunt and guarded but gradually reveals Mashiro's childlike personality. Kobeni adjusts cooking for Mashiro's preferences and supports her emotionally.
- Her friendship with Mayura Momouchi provides camaraderie, though Mayura's advice often proves unhelpful.

Her character development involves recovering fragmented memories of the cliff incident and reconciling her human existence with Hakuya's supernatural world. She learns to manage the physical repercussions of his powers, including fever episodes, while navigating high school life and societal perceptions. Progression from reluctant fiancée to committed partner highlights her emotional growth, particularly in asserting her feelings for Hakuya and establishing boundaries with Benio. By the series' conclusion, her acceptance of the arranged marriage transitions into a voluntary union, solidifying her integrated identity within both human and supernatural spheres.