|Family|| Mr. Kouno (father)|
Mrs. Kouno (mother)
Tooru Kouno (stepbrother/cousin)
Tooru's Father (uncle) †
Tooru's Mother (aunt) †
Even though Tooru is related to her, she's obsessed and possessive of him. She hates him talking and being with other girls so much that she once followed him on a date and attacked the girl he was with. Months after the incident, she found out where he was and went to his school to convince him to come back. When he rejected her feelings for him, she threatened to jump off a cliff if they couldn't be together.
Sayaka is the only daughter of Tooru's aunt and uncle that adopted him, making her Tooru's younger stepsister and also cousin. She is in love with Tooru and is very adamant, to the point of psychosis, about them getting married one day, against the wishes of Tooru himself (as his feelings for her never evolved beyond that for a sister). She is overly obsessive with him and has hurt one of Tooru's previous girlfriend by shoving her down a set of steps out of jealousy.
Sayaka searches for Tooru by coming to the school. Her fragile grip on reality was wounded by Yuujirou's claim that he and Tooru are lovers- and that Tooru is not interested in girls because of this. Then Yuujirou proceeds to kiss Tooru in front of her, and Sayaka ends up running away in shock.
Sayaka comes back on episode 11, where she begins to threaten Yuujirou by sending him a letter with a sharp razor blade on the top (that hurts Yuujirou's finger when he opens it), sending him numerous packages and asking some random guys to harass Yuujirou on the street. Mikoto suspected someone was harassing him, but Yuujirou tells Mikoto that he was okay and asked him not to mention anything to Tooru. However, Tooru began to suspect of Sayaka after the incident of Yuujirou with the guys on the street, and decides to have a firm talk with her to make Sayaka understand that he only sees her as a sister.
Later on, Tooru and the others discover Sayaka on the school again, but when Tooru try to call her she runs away. The boys run after her and Sayaka is cornered near a cliff, where she threatens to jump off if they come closer. Meanwhile, Akira goes to get the Student Council for help. A worried Tooru tries to reason with Sayaka, affirming that if she died not only he would be sad, but also his parents. Sayaka starts to cry and declares his love for Tooru once again, but Tooru tells Sayaka he will always see her as a sister and loved her that way.
Sayaka finally understand Tooru's feelings, but his foot slides and she falls with Tooru off the cliff. They are saved by the Student Council who, alerted by Akira, bring a mattress to cushion the fall. Sayaka then apologizes to everyone for all the problems she caused, and asks Yuujirou for his forgiveness. Before she goes, Sayaka tells she accepts Yuujirou as Tooru's lover and entrusts his care to him, much to the boys embarrassment, who affirm to the others that she has misunderstood the situation.
In vol. 3 of the manga, Sayaka is encountered again by Yuujirou and Tooru when they are out shopping, leading to Sayaka apologising, then declaring that in her mind only Yuujirou is worthy of dating Toru and he'd better hurry up and make her "brother" his.
- Sayaka has said that she prefers Yuujirou to be Tooru's boyfriend instead of a girl.
- In Princess Princess Plus, Yuujirou says that Sayaka still thinks he's Tooru's boyfriend, so she finally gave up on him.