Kouko Ibuki

Kouko Ibuki is the older sister of Fuko Ibuki who is a former art teacher at the Hikarizaka Private High School, and she treats everyone kindly. Kouko is one of the familiar faces in Furukawa Bread, the Furukawas' bakery, and Nagisa Furukawa admires her very much, being her former art teacher. She is married to Yusuke Yoshino (which now makes her Kouko Yoshino) and he is the major reason why Fuko still exists as an  in Tomoya Okazaki's school, despite still sleeping in the hospital.

Following her marriage, it is presumed she takes her husband's surname, which now makes her Kouko Yoshino. However, this name is not used in Clannad.

Clannad
During Clannad, we first see Kouko in the Furukawa Bakery as a client. We then learn she is Fuko Ibuki's sister. Kouko is engaged to Yusuke Yoshino, but she wants to wait until Fuko wakes up to marry him because she doesn't know if Fuko would have wanted her to. It is because of this reason that Fuko's giving out starfish shaped carvings to the students as invitations and is ignorant of her sister's doubts. She is invited by Tomoya and Nagisa to the founder's festival, and she attends, but she cannot see or hear Fuko.

A couple of days later, Kouko tells Nagisa Furukawa and Tomoya Okazaki about her sister, who was involved in a traffic accident and has been in a coma ever since. Kouko can't see Fuko, who is right beside them, but Fuko asks them not to tell her. Even though Kouko is ignorant of Fuko's existence as an ikiryou, she still believes that Fuko will someday recover. Tomoya and Nagisa manage to convince her to go on with the celebration since it would have been what Fuko would have wanted. A few days later, Kouko informs Nagisa and Tomoya about her sister's deteriorating health condition.

On the day of her wedding, she is granted permission to hold the ceremony in a classroom. At first, it seems like no one will come, but after she exits the building with Yusuke, she finds everyone who received a star from Fuko waiting for them. In the end, her sister's feelings managed to get across to everyone. Tomoya tells Kouko that all of this happened thanks to someone who wanted her to be happy. Kouko seems to realize that he is talking about his sister. Fuko then disappears, but not before greeting her sister one last time, wishing her happiness and handing her a star carving. Kouko thanks her and realizes that she took the right decision, and says that she will try to be happy for Fuko's sake too.

Kouko makes some brief appearances during the rest of the Clannad anime.

~After Story~
Kouko plays a minor role in the ~After Story~. She cheers Akio Furukawa's team in the first episode and she is also shown in Katsuki Shima and Yusuke Yoshino's flashbacks. A few years before, she was the only one who supported the latter when he started pursuing a musical career. She promised to wait for him after he came back. And when Yusuke finally returned, after having been forgotten by the public and getting involved in many scandals, she accepts him once again.

Kouko is also seen attending Nagisa's mock graduation ceremony and accompanying Fuko around after she woke up from the coma after being asleep for seven years. She is very worried about her sister's health, but Fuko feels that Kouko is too overprotective of her.

Movie
Kouko plays a greater role in the film than she does in the anime series. In the movie, she did not give up on teaching and was the drama teacher instead of the art teacher. She is also the former adviser of the drama club. She helps Nagisa Furukawa and Tomoya Okazaki to form the Drama Club again and prepare a play. She is also shown visiting them at their house after they get married and accompanies Tomoya to meet his daughter. In the movie, she practices Judo and uses it to stop an overly aggressive Youhei Sunohara.

Etymology

 * The name Kouko means "public" (公) (kou) and "child" (子) (ko).
 * Kouko's surname Ibuki means "that one" (伊) (i) and "blowing" (吹) (fuki/buki).
 * Her married name Yoshino means "fragrant, virtuous, beautiful" (芳) (yoshi) and "field, wilderness" (野) (no).