Choco is the central character of this narrative, introduced as the answer to a childhood wish made by a young man named Haruma Kawagoe. Having always wanted a baby sister, Haruma's request was eventually fulfilled years later when a woman claiming to be Santa Claus delivered Choco to his home on a flying motorcycle. Upon her arrival, Choko has no name of her own and asks Haruma to give her one; he names her Choko, a word that refers to the Japanese term for a manual or cheat sheet. This name is a direct reference to the instruction manual she carries, which contains guidelines on how to behave as a proper little sister.

Choco presents herself as a sweet, very innocent, and childlike girl who appears to be approximately ten years younger than Haruma, placing her in the age range of nine to twelve years old. At the start of her story, her entire understanding of the world is derived from her instruction manual, leading to a unique and often naive perspective on daily life. The manual does not cover every situation, and some of the information it provides can be inappropriate for someone of her apparent age, contributing to humorous or awkward misunderstandings. Choco possesses an irrepressibly kind and trusting nature, refusing to believe that anyone or anything she likes could do something bad.

Her primary motivation is her deep and unwavering affection for Haruma, whom she immediately calls onii-chan, a term for an older brother. She is always eager to be the most important person in his life, although over time she learns that she cannot be his favorite in every situation. Her role in the story is to be a new and transformative presence in Haruma's previously quiet life, forcing him to confront his loneliness and take on the responsibilities of a caretaker, even as his finances are strained by the need for part-time jobs. Despite the magical circumstances of her arrival, a significant part of her existence involves performing everyday activities like shopping and keeping house while Haruma is away at university.

Choco forms several key relationships with the people she meets. Her most important bond is, of course, with her brother Haruma. She also befriends children her own age, including Kakeru Ishida, a shy boy who develops a strong crush on her after accidentally saving her from a fall, and Yurika Hanayamada, a wealthy girl who initially dislikes Choco but eventually grows to care for her deeply despite having trouble expressing her feelings. Within Haruma's apartment building, she interacts with the other tenants, such as the accident-prone landlady Chitose Serikawa, whom Choco eventually accepts as her brother's romantic partner.

Choco's character undergoes notable development when she confronts the nature of her own existence. A pivotal moment occurs when Haruma denies that she is his little sister while speaking to a visiting cousin, causing Choco to disappear. This event forces Haruma to truly remember and acknowledge the childhood wish that brought her into being, and he prays for her return. After this incident, she also experiences hidden insecurity, occasionally wondering if Haruma might grow bored of her and decide to discard her. Through such experiences, she moves from simply following the instructions in her manual to developing a more nuanced emotional understanding and a secure place within her new family.

As for notable abilities, Choco is not a typical human girl, though the exact nature of her creation is not fully explained. The Santa figure who delivered her notes that creating a little sister takes much more time than making an android, implying a more organic or magical origin for Choco. Her most defining ability is her capacity to learn and adapt to human society using her instruction manual as a foundational guide. She also has a powerful, almost magical effect on the people around her, bringing warmth and change into their lives simply through her presence and innocent kindness.