Kimihito Kurusu is a normal young Japanese student living in Asaka, Saitama, in his parents' home while they work abroad. He holds a part-time job and was involuntarily thrust into the role of a host family for the Interspecies Cultural Exchange Accord due to an administrative error. His given name is rarely used by other characters, with the monster girls in his care affectionately calling him by various pet names such as Darling, Boss, Master, Beloved, or Honey. Physically, he is a tall young man of average appearance with messy black hair, often depicted with simple white circles for eyes that reveal brown irises only during moments of genuine seriousness or emotion. His body is frequently covered in scrapes, bruises, and bandages as a result of the daily hazards of living with his supernatural housemates.

In terms of personality, Kimihito is defined by his profound kindness, patience, and selflessness. He treats the liminals, or monster girls, not as exotic or dangerous creatures but as ordinary people deserving of respect and dignity. This unconscious ability to see the normal girl within each extraordinary being allows him to befriend liminals with remarkable ease. Despite not having volunteered for the program, he accepts his chaotic situation with good grace and a long-suffering nature, enduring constant physical mishaps and the manipulations of the exchange coordinator, Ms. Smith. He rarely displays anger, but when he does it is notably assertive, such as when he defended Miia from a racist couple or insisted that Lala not play with her food. His kindness can sometimes border on the strange, as he remains unfazed by kidnapping attempts and is more concerned with the health and comfort of his would-be captors than his own safety.

Kimihito's primary motivation is the well-being and protection of the liminals he considers family. He repeatedly and unhesitatingly places himself in physical danger to shield them from harm, whether by stepping in front of a sword meant for Centorea, diving into water to save an injured Mero, or nearly being absorbed by a giant slime while trying to rescue Suu. This protective instinct stems from his genuine care for the girls, not any sense of obligation. He is further motivated by a desire to see them happy and comfortable, going to great lengths to accommodate their unique needs, from remodeling his home to studying specific ecologies like that of snakes to better care for Miia.

Kimihito serves as the central protagonist and the stable anchor for the entire household. His role is that of the long-suffering caretaker and mediator, maintaining peace among the increasingly large and competitive group of monster girls. He is the primary point of contact for the cultural exchange program, and his home is the setting for nearly all of the story's events. The narrative revolves around his daily life managing the romantic advances, jealousies, and cultural mishaps of his housemates, as well as his later designation as a test case for new interspecies marriage laws, which forces him to consider choosing one of the girls as a bride.

His key relationships form the core of the story. With Miia, the lamia who first moved in, he shares a deep bond; she is intensely possessive and loving, calling him Darling, and he cares for her immensely, once punching a man for insulting her. Papi, the harpy, is childlike and dim-witted, viewing him as both a big brother and a lover, though Kimihito primarily sees her as a younger sister. Centorea, the centaur, treats him as her master and sworn lord, developing a loyal and chivalrous love for him after he protected her. Suu, the slime, is similarly viewed as a child or little sister, and she adores him for his kindness. Meroune, the mermaid, possesses a dramatic obsession with tragic love and refers to him as her Beloved. Rachnera, the arachne, is cynical and flirtatious, calling him Honey and finding amusement in toying with him, though she genuinely loves him for seeing her spider legs as charming. Lala, the dullahan, is fascinated by his repeated escapes from death and stays to observe him.

Kimihito undergoes notable development throughout the series. Initially just an ordinary student unwillingly hosting a single liminal, he grows into a confident, though perpetually exhausted, head of a diverse household. His indecisiveness regarding the romantic affections of the seven girls is a recurring challenge, but he strives to make a definitive choice to ease their anxiety. His mental fortitude, described by Lala as so strong that it allows him to return from near-death states, becomes a defining trait. He also actively trains his body, acknowledging that he needs to become physically and emotionally stronger to survive a committed relationship with an extraspecies woman.

Kimihito possesses several notable abilities that set him apart. He is an exceptional cook, capable of creating gourmet-quality meals from limited or unusual ingredients, a skill that has caused his housemates to gain weight. He is also highly proficient with a needle and thread, regularly altering and repairing clothes for his liminal housemates. He has an indomitable will and remarkable physical endurance, surviving daily beatings, poisonings, and accidents that would kill a normal person, bouncing back with little consequence. He also demonstrates surprising strength, such as moving both Miia and her mother, whose combined weight is considerable. Another unique, though unwillingly acquired, talent is his masterful hand-milking technique for bovine and caprine liminals like minotaurs and pans, a skill that becomes highly demanded. His willpower extends to resisting the powerful sexual advances of the monster girls, allowing him to maintain his composure even during a full moon when their primal instincts are at their peak.