Kasumi Haruno, commonly called Kasumin by those around her, is a young fourth-grade girl. Her parents are zoologists who often travel for their research, and at the start of the story, they depart for an expedition to Africa, leaving Kasumi in the care of the Kasumi family at their large, mysterious mansion. Despite her young age, Kasumi demonstrates a high level of self-sufficiency and maturity. This independent nature is a direct result of her parents' frequent and prolonged absences due to their work, which has forced her to learn how to take care of herself from an early age. Ironically, while Kasumi remains calm and level-headed about each new journey her parents take, they are often the ones who struggle with saying goodbye, worried about their daughter's wellbeing even as she comforts them.

Kasumi is consistently depicted as kind, polite, and remarkably grounded. Her personality is one of her greatest strengths, especially as she finds herself in an entirely unusual living situation. Upon arriving at the Kasumi Mansion, she quickly discovers that the family she will be living with is not human. They are Henamon, a type of creature with supernatural abilities who can transform at will and control a specific element. Furthermore, many of the household appliances and objects within the mansion are alive and can speak. Rather than being frightened or overwhelmed by this strange new reality, Kasumi adapts with a calm resolve. She agrees to help with household chores, including cooking and cleaning, in exchange for room and board, taking on a role similar to a maid within the household. Her ability to navigate this world of magical beings with only her human qualities, particularly her unwavering determination, is a central aspect of her character. She has no magical powers of her own, making her persistence and resourcefulness all the more important for surviving and building relationships among the Henamon.

Kasumi's central role in the story is to be the ordinary human protagonist living among extraordinary creatures. Her presence serves as a bridge between the human world and the secret world of the Henamon, whose existence has been hidden from humanity for centuries. She is given the nickname Kasumin by Ryunosuke, a member of the Kasumi family, and this nickname is used by Sakurame and other inhabitants of the mansion. Her primary motivation is adapting to her new life, fulfilling her duties in the household, and navigating the comedic and sometimes challenging situations that arise from living with such unique characters. As she interacts with the Kasumi family and other Henamon, she builds strong friendships and becomes an integral part of their community. Her relationships are key to the story's development, as the series focuses heavily on the daily life interactions between Kasumi and these charismatic creatures. Her ability to accept them for who they are helps to deepen the Henamon's understanding of humans as well.

A notable personal skill of Kasumi is her cooking ability, which is described as quite good for a girl her age. This talent is a practical manifestation of her self-reliance. Her character undergoes natural development through her experiences in the Kasumi Mansion. While she arrives as a girl already accustomed to solitude and independence due to her parents' work, living with the Henamon exposes her to a different kind of family dynamic. She learns to live among non-humans, keep their secrets, and form bonds that are not solely defined by her human background. Her growth is not about gaining magical power, but about deepening her emotional resilience and her capacity to create a home and a family in the strangest of circumstances. Her unwavering spirit and her willingness to embrace the unknown are her most notable abilities, allowing her to face the various magical beings and situations she encounters with courage and a practical mind.