Sayaka Kouno

Sayaka Kouno (河野 さやか Kōno Sayaka) is Tooru's younger stepsister and cousin.

Personality
Even though Tooru is related to her, she is 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 Tooru was in Fujimori Academy and went to the school to convince him to come back home. When he rejected her feelings for him, she threatened to jump off a cliff if they couldn't be together.

Background
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) and her parents. 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.

Anime
Sayaka searches for Tooru by coming to the school. Her fragile grip on reality was wounded by Yuujirou's claim that he and Tooru were lovers- and Tooru is not interested in girls because of this. Then Yuujirou proceeds to kiss Tooru in front of her, which ends with Sayaka 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 paying some random guys to harass Yuujirou on the street. Mikoto suspected someone was harassing him, but Yuujirou tells Mikoto that he was okay and begs him not to mention anything to Tooru. However, Tooru began to suspect of Sayaka after the incident on the street, and decides to have a firm talk with her to make Sayaka understand he only sees her as a sister.

Later on, Tooru and the others discover Sayaka on the school grounds again, but when Tooru tried to call her she runs away. The boys chase after her and Sayaka is cornered near a cliff, where she threatens to jump if they come closer. Meanwhile, Akira goes to 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 their parents. Sayaka starts to cry and declares her love for Tooru once more, but Tooru tells Sayaka that he will always see her as a sister and loved her in that way.

Sayaka finally understands Tooru's feelings, but her foot slides and she falls with Tooru off the cliff. Both are saved by the Student Council who, alerted by Akira, brought 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 returns home, Sayaka says 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.

Manga
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.

Etymology

 * The name Sayaka is written in hiragana (さやか), which has no special meaning. However, when written in kanji (沙也香, 沙耶香, 沙也加 or 紗耶香), it could possibly mean:
 * 沙也香 - "sand" (沙) (sa), "to be, also" (也) (ya) and "fragrance, fragrant" (香) (ka).
 * 沙耶香 - "sand" (沙) (sa), "question mark" (耶) (ya) and "fragrance, fragrant" (香) (ka).
 * 沙也加 - "sand" (沙) (sa), "to be, also" (也) (ya) and "add, increase" (加) (ka).
 * 紗耶香 - "thread, silk" (紗) (sa), "question mark" (耶) (ya) and "fragrance, fragrant" (香) (ka).
 * Sayaka's surname Kouno means "river, stream" (河) (kou) and "field, wilderness" (野) (no).

Trivia

 * 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.