Aya Arisugawa is the older sister of Kaname Arisugawa and one of the residents of the bathhouse that Nasa Yuzaki frequently visits. As a high school student of about seventeen years old, she is recognized for her gentle beauty, characterized by very long periwinkle hair and purple eyes, which has led to her being considered quite popular.

Her personality presents a fascinating contrast to that of her younger sister. While Kaname is often portrayed as the more responsible and perceptive one, Aya is generally easygoing and can be seen as a lazy ditz. However, Kaname herself considers Aya to be on the genius side due to her great capacity for learning, suggesting that her lack of focus is a matter of choice rather than ability. This dichotomy is central to her character; she can grasp new concepts and skills with remarkable speed, yet she frequently misunderstands important social cues and becomes confused in the blink of an eye. Her favorite hobby is playing video games, and she often nags Kaname into playing with her. She is also easily influenced by television programs, sometimes developing sudden and fleeting passions for things she sees on screen, such as wanting a garden pond after watching a show about them.

Aya's motivations and role in the story are deeply intertwined with her long-standing feelings for Nasa Yuzaki. She first became aware of him during a middle school math class when he effortlessly solved a problem that no one else could, and she fell in love with him at that moment. This crush continued for years, becoming a significant part of her life. Despite these feelings, she never found the courage to confess to him. Her role in the narrative is initially defined by this unrequited love, which comes to a head when she inadvertently discovers that Nasa has married Tsukasa. Her mother reveals Aya's secret feelings in front of everyone, leading to a moment of heartbreak. Instead of becoming bitter or trying to sabotage the relationship, Aya makes the mature decision to not sulk and to congratulate the couple, choosing to support them rather than let her first love become a source of negativity.

Key relationships shape her development throughout the series. Her relationship with her sister Kaname is close, with Kaname often acting as the more grounded sibling who can easily see through Aya's attempts to hide her emotions. The most significant relationship development, however, is with Tsukasa Yuzaki. Initially, Aya has trouble accepting Tsukasa as Nasa's wife and feels that she herself might be more suited for him. This leads to a confrontation during a takoyaki party, where Aya challenges Tsukasa to a video game battle to test her. Aya is extremely skilled at video games, winning twenty rounds of Street Fighter V in a row. However, when they switch to a different game, Tsukasa starts winning, and they exhaust themselves playing until one in the morning, eventually calling it a draw. Through this shared passion for gaming, Aya and Tsukasa develop a genuine bond of friendship and mutual respect. Aya affectionately begins to call her Tsukacchan, and they become close friends and friendly rivals. She also has a combative relationship with Ginga Onimaru, Nasa's cousin, with the two frequently bickering and showing open rivalry.

Aya possesses several notable abilities, the most prominent being her exceptional talent for video games. She is a game otaku of a near-professional level, capable of picking up new games and quickly reaching a high level of skill, even rivaling experienced players. She channels this talent into her online persona, Ayapi, where she has become a popular streamer and YouTuber, turning what might have been a simple hobby into a significant and successful pursuit. Beyond gaming, Aya has developed a remarkable resistance to heat from excessive use of her family's sauna, earning her the unofficial title of a heat wave master. This contrasts with her lack of dedication to academics, as she frequently skips classes and is in danger of having to repeat a school year due to her poor attendance, prioritizing her hobbies and streaming over her studies.

Her character development is one of graceful acceptance and redirection. After the initial shock of losing her chance with Nasa, she does not dwell on her disappointment. Instead, she uses the experience to propel herself forward, investing her energy into her streaming career and forging a new, meaningful friendship with Tsukasa. She demonstrates emotional maturity by supporting the couple and becoming one of their closest friends, proving that she is not a typical bitter rival but a resilient and good-hearted person who chooses happiness and connection over resentment.