Mafuyu Hoshikawa is a waitress at Cafe Stile, where she performs the role of the cheerful and affectionate "little sister" for the customers. Despite her youthful and petite appearance, she is actually a college student in her early twenties, making her one of the oldest and most mature members of the staff. Her small stature, which includes a very short height, is the primary reason she was assigned the imouto character type, as she naturally fits the visual image of a younger girl.

In her daily life away from the cafe, Mafuyu has a reserved and composed demeanor. She rarely displays strong emotions and typically maintains a calm, expressionless face. This quiet personality contrasts sharply with the lively, sweet, and energetic little sister persona she adopts while working. Her true, more serious nature tends to emerge when she is irritated, particularly by the antics of her manager, Dino. She does not hesitate to scold him or even resort to physical reprimands, such as hitting him on the head, when his laziness or obsession with anime interferes with his work.

Mafuyu has a deep and passionate love for a children's magical girl anime series called Magical Frill. This interest is a significant part of her private life, and she becomes visibly excited and enthusiastic whenever the show or related merchandise is involved. Her devotion to the series is so strong that she will prioritize watching it over almost any other activity. This passion also leads to conflict, most notably with Dino, whom she considers a fickle fan who changes his favorite character every season. On one occasion, she resorted to punching him to win a Magical Frill figurine they both wanted.

Her appearance is characterized by her short, chin-length brown hair and purple eyes. She is frequently mistaken for an elementary school student due to her small physique, which is a source of frustration for her, especially when she is told that drinking beer looks inappropriate for someone of her height. She is also somewhat envious of taller, more developed women, such as her coworker Kaho, stating that she could never convincingly play the little sister to someone with such a mature figure. Conversely, she is very happy when young children call her "big sister," likely because she has a younger brother who is taller than her.

Within the story, Mafuyu serves as a grounded and level-headed presence among the cafe staff. She is often the voice of reason and helps to keep her coworkers in line. She has a key relationship with the manager, Dino, who recruited her after a strange encounter in a park. She treats him with a unique blend of respect and exasperated authority, often acting as his supervisor to ensure the cafe runs smoothly. She also has a mentoring relationship with the newer waitress, Kaho, helping her with her studies and guiding her at work, though she can be strict in her teaching methods.

Over the course of the series, Mafuyu shows little change in her core personality but reveals more of her hidden depths and softer side. She develops a unique friendship with a regular customer named Itou. Although he expresses romantic interest in her, she is initially hesitant and prefers to keep their relationship platonic. This friendship, which remains close even years later, is significant as it is one of the few personal relationships she cultivates outside of work. She is also shown to be skilled with her hands, crafting small gifts for visitors, and is athletic, demonstrating surprising talent in snowboarding. Her long-term aspiration is to work with children, aiming to become an elementary school teacher or a childcare worker, a goal that aligns perfectly with her natural affinity for kids and her nurturing yet firm personality.