Aoi Wakatsuki is the central protagonist of the series, introduced as a first-year student at Fujimi High School who joins the prestigious tennis club. Initially, she lacks confidence and is portrayed as an ordinary, somewhat clumsy girl with no special athletic talent, often feeling out of place among her more experienced and naturally gifted peers. Her background is relatively simple, showing a young girl who is gentle and somewhat introverted, but who possesses a deep, hidden reservoir of determination and a pure love for the sport.

In terms of personality, Aoi is defined by her sincerity, kindness, and emotional vulnerability. She is prone to crying under pressure and often doubts her own abilities, especially when compared to her rival, the elegant and talented Reika Ryuzaki. Despite this, Aoi is not fragile; she possesses a remarkable capacity for hard work and endurance. Her humility allows her to learn from everyone around her, from her strict coach to her senior teammates, and she never gives up no matter how insurmountable the challenge seems.

Aoi’s primary motivation evolves as the story progresses. Initially driven by a simple desire to improve and avoid letting her team down, she later becomes fiercely motivated to live up to the harsh but profound expectations of her coach, Jin Munakata. Her goal shifts from merely hitting the ball correctly to understanding the true soul of tennis. She strives to become a player worthy of her coach’s sacrifices and to one day defeat her rival, Reika, in a fair match.

Her role in the story is that of the underdog who undergoes a grueling physical and psychological transformation. Aoi is the lens through which the intense world of competitive tennis is explored, highlighting themes of perseverance, grief, and the painful beauty of mastering a craft. Her journey from a novice to a dedicated athlete forms the core narrative.

Key relationships profoundly shape her development. Her relationship with Coach Jin Munakata is the most significant; his stern, often seemingly cruel teaching methods push Aoi to her limits, but his underlying faith in her potential becomes her greatest source of strength. Her rivalry with Reika Ryuzaki is complex, moving from simple jealousy and admiration to a deep, mutual respect that pushes both players to higher levels. Her friendship with her doubles partner, Kyoko Otowa, also helps ground her, providing emotional support and practical partnership on the court.

The development of Aoi Wakatsuki is the heart of the series. She transforms from a weeping, uncertain beginner into a resilient and focused athlete capable of immense concentration and fighting spirit. This growth is often painful, involving extreme training and the emotional trauma of losing her coach. By the end of her story, she has not necessarily become the most technically perfect player, but she has forged an unbreakable will, learning to channel her emotions into her game rather than being crippled by them.

Regarding notable abilities, Aoi does not start with any natural gifts. Her primary ability is her extraordinary stamina and her capacity for relentless repetition and training, which she develops under Coach Munakata’s guidance. She becomes known for her sliding footwork, often referred to as the "dog run," a desperate but effective way to reach every ball. Her signature shot is a powerful, albeit initially unpolished, overhand smash or serve, which she works tirelessly to improve. Ultimately, her greatest ability is her mental fortitude, the refusal to surrender a single point, which allows her to turn matches around through sheer persistence.