Hakuya Mitsumine belongs to the non-human Inugami clan, originating from a rural mountain village. Due to an ancestral pact, his family arranged his engagement to Kobeni Yonomori. He and his younger sister Mashiro relocated to the Yonomori household upon Kobeni's sixteenth birthday, initiating their cohabitation.

A critical childhood event forged his bond with Kobeni. During a visit to his village, he failed to prevent her fall from a cliff. Rescuing her caused a severe injury to his right eye, resulting in permanent vision impairment and a scar hidden beneath his long bangs. To save her life, he transferred half of his Inugami powers into her body. This act caused lingering incompatibility, manifesting as occasional fevers and a weakened constitution for Kobeni, while leaving him with chronic fatigue requiring extended sleep to conserve energy. The incident also erased Kobeni's memory of their shared past.

Physically tall with spiky black hair and dark purple eyes, he possesses enhanced speed, endurance, and agility, enabling feats like leaping vast distances. He can shapeshift into a black dog but consciously avoids doing so. His hobby involves constructing intricate historical building models from matchsticks, though Mashiro frequently damages them.

His personality is reserved and stoic, exhibiting minimal facial expressiveness that makes his emotions difficult for others to interpret. Exceptions include Kobeni, who can discern his feelings, and his mother Shirayuki. He displays significant patience and self-control, tolerating behaviors like Benio's aggression or Mashiro's tantrums without reaction. Generally disinterested in sweets, he consumes them when prepared by Kobeni. He is protective of both Kobeni and Mashiro, often following Kobeni discreetly to ensure her safety. Socially, initial wariness from classmates due to his demeanor gradually shifts as they recognize his athletic skills and deadpan humor.

Key relationships define him:
- **Kobeni Yonomori:** He demonstrates unwavering loyalty and deep affection, viewing any gesture from her as significant. The idea of her disliking him causes severe distress. Their relationship evolves from initial awkwardness to mutual dedication, culminating in marriage.
- **Mashiro Mitsumine:** As her older brother, he balances indulgence with discipline, ensuring her well-being. Mashiro interprets his subtle cues for others and advises him on understanding Kobeni. She relies on him for protection, particularly from Benio.
- **Benio Yonomori:** He endures her hostility stemming from jealousy over Kobeni and Mashiro, though he finds her actions distasteful. Their interactions remain tense except when discussing Kobeni, where they share common ground. Benio privately acknowledges gratitude for his life-saving intervention.
- **Shirayuki Mitsumine:** He respects his mother despite her tendency toward thoughtless revelations. He assists her with chores and cares for Mashiro in her stead.

His name translates to "white night," reflecting his heritage. Relatives Arata and Ui Komagamine refer to him affectionately as "Ha-kun."