Haruma Kawagoe is the main protagonist of the story and is introduced as a university student living on his own. The circumstances that lead to his unique situation are rooted in a childhood tragedy. When Haruma was young, his mother became pregnant with a baby sister he eagerly anticipated. She suffered a miscarriage, and the complications from the event led to a hysterectomy to save her life, making it impossible for her to have more children. In response to this loss, a young Haruma prayed for his mother's recovery and made a heartfelt Christmas wish, asking Santa Claus to deliver a little sister to him in place of his mother. This wish is seemingly forgotten until years later when, on Christmas morning, a tough-looking woman on a flying motorcycle claiming to be Santa arrives at his window to deliver the requested sibling, a young girl who comes with her own instruction manual.

As a character, Haruma is portrayed as an ordinary and relatable young man who is suddenly thrust into an extraordinary and unexpected responsibility. Despite the absurdity of the situation, he demonstrates a strong and immediate sense of care for the girl he names Choko. He is generally kind, patient, and protective of her, taking on a nurturing role despite his initial lack of preparation. He lives in a modest manor and, being a college student without a stable income, he frequently works part-time jobs to support himself and his new charge, showing a practical and hardworking side to his personality. While he is often level-headed, he also experiences the typical romantic feelings of a young man, initially harboring a crush on Ayano Sonozaki, a local flower shop owner.

Haruma's primary motivation throughout the story is to be a good older brother and provider for Choko, the little sister he always wanted. He feels a deep-seated obligation to care for her, which stems from his childhood wish and the void left by the sister he never had. He accepts the mysterious circumstances of her arrival with relative calm and focuses on integrating her into his daily life, teaching her about the world and protecting her from its dangers. His role in the narrative is that of a guardian and an anchor, not only for Choko but also for the growing circle of people around him, including the residents of his boarding house and his university acquaintances.

Key relationships define much of Haruma's development. His most significant bond is with Choko, whom he treats as a real sister despite her strange origin. He is patient with her naivety, is proud of her cute personality, and is determined to help her learn and grow. His romantic life also plays a crucial part in his characterization. He has a one-sided affection for Ayano Sonozaki, but his hopes are ultimately dashed when she reconciles with her former fiancé, leaving Haruma heartbroken. This experience allows him to grow and later open his heart to Chitose Serikawa, the accident-prone but kind-hearted granddaughter of his landlady. Haruma helps Chitose gain confidence in her appearance, which causes her to develop a crush on him. Over time, he comes to realize his own feelings for her, confessing his love after she attends an arranged marriage meeting.

In terms of personal development, Haruma begins as a typical young man living alone, focused on his studies and personal life. The arrival of Choko forces him to mature quickly, taking on financial and emotional responsibilities that are almost parental in nature. He transitions from someone on the receiving end of care to a primary caregiver, learning to put someone else's needs before his own. His romantic journey also shows his growth, as he moves from a superficial crush to a more mature, reciprocated love with Chitose. In the manga, this relationship culminates in marriage, as Choko later refers to Haruma and Chitose as her big brother and big sister in a manner that implies a settled family life together.

Regarding notable abilities, Haruma does not possess any superhuman powers or special talents. His strengths are grounded in his character. He has the ability to remain calm and adapt to the most illogical and surprising situations, such as accepting a new sister delivered from the North Pole. More practically, he is shown to be resourceful and hardworking, consistently finding ways to earn money through part-time jobs to maintain his meager household and care for Choko. His most significant ability is his capacity for deep, compassionate caregiving, as he successfully navigates the challenge of raising and nurturing a young girl with no prior experience or warning.