Hiromi Oka is the central protagonist of the story, a high school student whose life is transformed when she discovers tennis. Initially, Hiromi is a shy, sensitive, and unassuming fifteen-year-old with very little confidence in her own abilities. She is prone to tears and often feels overwhelmed by the pressures of her new environment. Despite this fragility, she possesses a deep reservoir of inner strength and a genuine, kind-hearted nature, always remaining a nice girl even toward those who are hostile to her. Her clumsiness and naivety make her an unlikely candidate for a sports champion, but it is precisely this contrast that defines her early journey.

Hiromi's primary motivation stems from her passionate admiration for Reika Ryuzaki, the team's star player known as Madame Butterfly. Watching Reika's graceful and powerful playing style inspires Hiromi to join the tennis club, initially wanting to emulate her idol. However, her motivations evolve significantly over time. While she starts by playing tennis to be like someone else, her constant struggles, failures, and the intense training imposed by her coach force her to find her own reason to play. She eventually develops a profound and genuine love for the sport itself, making it a core part of her identity.

Within the story, Hiromi serves as the ultimate underdog. She joins the prestigious Nishi High tennis team with little skill and is initially disliked and bullied by her own teammates, who view her as an undeserving teacher's pet. Her role is to embody the spirit of perseverance. She faces overwhelming odds, including a coach who employs training from hell, fierce rivals, and her own self-doubt, yet she continues to strive for excellence. Her mission is to prove that hard work and determination can overcome a lack of natural talent and the scorn of one's peers.

The key relationships in her life are central to her development. Her bond with Coach Jin Munakata is the most significant. Despite his stoic and stern demeanor, he is the first to see her hidden potential and dedicates himself to molding her into a great player. This relationship is built on tough love and deep mutual respect, and his faith in her becomes a cornerstone of her determination. Her rivalry with Reika Ryuzaki evolves from one-sided admiration to a tense, respectful competition that pushes both players to their limits. She also finds support in her best friend, Maki Aikawa, who stands by her during the toughest times, and a gentle romantic interest in Takayuki Todo, a member of the boys' tennis team.

Hiromi undergoes a remarkable transformation from an extreme doormat who lets others walk all over her to a self-possessed and formidable athlete. She experiences several "break the cutie" moments, including complete devastation after a personal tragedy, but she always finds a way to continue. Her development is a slow, steady climb; she loses many matches and faces humiliating defeats, but each failure teaches her a lesson about the mental and emotional aspects of tennis. She learns to stop running from her anxieties and to face the loneliness of the court, ultimately becoming a player who can stand as an equal to the very people she once worshipped.

In terms of notable abilities, Hiromi is not a prodigy. Her greatest asset is not a special technique but her determination and capacity for hard work. She is a plucky girl who takes every harsh lesson from her coach and turns it into a physical and mental skill. Through relentless practice, she gradually develops her tennis instincts, strategic thinking, and the physical stamina required to compete at a high level. Her growth is marked not by the sudden acquisition of a signature move, but by a steady improvement in her fundamentals and, most importantly, her mental fortitude. She learns to throw away distractions, stay focused, and transform her love for the game into a powerful, competitive weapon.