Abandoned at birth in a rural town in Osaka, Sayo was passed to different caretakers up until Miaki's family moves into town and her father tracking down his missing niece. By that time, Sayo is adopted to the Tsukishima family and has become Miaki's little sister. Since then, she is treated like family, as Miaki develops a fondness for her cheerful bundle of joy.

When Miaki began helping out with Shiraishi, Sayo develops an extreme interest in tennis and begs her family for her to get her own gear (which they agreed to do so) She becomes some sort of a mentee/student for Shiraishi and Kenya while Miaki helps her out in her training routine. It becomes apparent that Sayo has natural talent for tennis and is a bit of a genius when it comes to absorbing most of what her mentors would teach her. It has come to the point in which she feels like she is stagnating there in Shitenhouji when it comes to finding more tennis styles to learn from. Upon hearing her sister talk about her friend in Rikkaidai, she applies for a student transfer/exchange program and makes it in, thereby starting her second year in Rikkaidai.

Despite being accompanied by her sister when she first arrives in Rikkai, Sayo still got lost and ended up in the tennis courts. There, she meets Kirihara doing some basic trainin exercises. Sayo wastes no time in asking him for a match, which Kirihara agrees in the end just to get her off his back. He expected a quick beat down (and even suggested that the match would only be in three games) but was thoroughly surprised after the first game (which he knew she threw in favor of analysing him) She changed the tide of the match somehow as she shows off some of the foundations and styles she developed using the Shitenhouji tennis club members as her basis.

As the battle gets heated, the rest of the regulars appeared, commentating on her various techniques and unorthodox play styles. In one instance, Yanagi even comments that her style is completely targeted at those who excel in reading their opponents as she exploits various habits and subconscious decisions of the other team. When she scores another point with her Mantis stalking Cicada move, Kirihara nearly goes into Devil Mode but was stopped by Sayo doing something (explained by Yanagi as a sensory overload type of Yips) The game ends with Sayo winning as she runs off to the main building, lamenting that her nee-san will be mad at her. When she appears the next day in Kirihara's class, he talks to her, clearly intrigued by her tennis prowess. It is the start of an unexpected friendship.

Compared to her cousin/sister, Sayo is much more energetic and less likely to be held down by the rules if it hinders on what she wants/needs. Though she is considered as a tennis prodigy, she seems to dislike the title, as she is more interested in uncovering and implementing different tennis style to her own. She is nosy and prone to be a bit affectionate due to her natural curiosity and unsatiable need to discover everything around her. At some point, Yanagi even describes her as someone who is very gluttenous towards every piece of knowledge she could get her hands on.

Sayo is also rarely ever angry or insulted by anything. Though she does get into her fair share of fights due to her blunt nature. She doesn't dwell on loses or wins, just the experience and knowledge she learns. This does mean that she hates default wins/loses or sweep matches as she is not able to decipher more about other people's tennis styles. Though people in Shitenhouji used to describe her as a female Kintaro, she is much more focus and goal-oriented than Kintaro's wild style. This is especially true when Kirihara accompanies her to one of her Calligraphy sessions as he notes how she looks as if she is in a trance.

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