Isahaya is a nice girl most of the time, but is capable of being very nasty when she feels jealous. She also has a mentoring side, as she watches out for her underclassman Ariake after she gets over her initial dislike of the younger girl.
Isahaya is left handed. Isahaya wears the standard girl's uniform of Hakoniwa Academy, as well as the uniform of the Athletics Club. She has dark green eyes and brown hair (black in the anime) tied in a short ponytail and held with a clip. At the Seiyasai, Isahaya wears a light colored scarf with her uniform. Ten years later, Isahaya wears an apron over a white shirt and black pants.
Student Council Executive Arc
Isahaya attacks Ariake after she is made a regular at Isahaya's expense, vandalizing her shoes and leaving a note in her locker saying "Quit the Athletics Club". After Medaka Kurokami and Zenkichi Hitoyoshi start investigating at Ariake's request, Hansode Shiranui points them in Isahaya's direction. Medaka confronts the older girl directly about wether or not she vandalized Airake's shoes. Isahaya runs away, with Medaka following closely behind, and ultimately overtaking her by jumping over her head and landing in front of her. Medaka then asks her once again whether she is the culprit. When Isahaya rather poorly denies it, Medaka takes her word for it, much to the runner's surprise. She then leaves, also complimenting Isahaya on her running. As Medaka leaves, Isahaya questions whether she knows how to doubt people. Zenkichi tells her that while Medaka knows how to doubt, she trusts people first, and tells Isahaya that he'll follow Medaka's lead, and believe that she'll never do something like this again. Isahaya falls to her knees in tears. The next day, Ariake comes to the Student Council room to tell them that the sneakers she was using to train with were replaced with new running shoes and a note saying "Sorry".
At a later date, Isahaya is seen running with Ariake. Isahaya scolds her for taking too long to get changed, and if they don't hurry they will miss morning practice. Ariake apologizes, but questions why Isahaya waited for her, commenting that she has been a lot nicer to her lately. Isahaya insists she hasn't, and not paying attention, crashes into someone, though she is caught and dropped gently on the ground. An embarrassed Isahaya apologizes, to which the other party tells her to be more careful, as he would hate to see her get hurt. When Ariake asks if she is all right, a flustered Isahaya asks who that was. Ariake identifies him as Kouki Akune, known as the prince of the Judo Club.
While looking for a new model for Kizashi Yuubaru, Zenkichi approaches Isahaya, who he convinces to come with him through a combination of blackmai and flattery. After changing, she models for the artist along with Nekomi Nabeshima, though both are rejected. After Yuubaru rejects Zenkichi and Akune as well, she accuses him of being too picky. When Yuubaru decides on Hansode Shiranui, she is notably shocked with everyone else, and becomes depressed along with the other girls when Yuubaru decides she is perfect because her body is underdeveloped.
At the club battle swim meet, Isahaya is seen as one of the participants representing the Athletics Club, along with Ariake. Isahaya and Ariake take part in the second event, the three legged race. When Zenkichi and Akune take the lead in the race despite their poor teamwork, Isahaya is seen looking surprised with Ariake. Isahaya even comments that the Athletics Club is losing face. She is then surprised to see that the Swimming Club is in last place. Both she and Ariake are seen shocked along with everyone else when Umumichi Yakushima and Sotsu Tanegashima begin swimming with their legs tied together, taking first place. The Athletics Club comes in second place. At the end of the third event, the Athletics Club is in second place overall.
After Medaka crash-lands outside of the school, she asks Isahaya for her cleats, to which Isahaya concedes. As she is left standing with Medaka's destroyed shoes, Ariake wonders what Medaka did to destroy her old shoes, which Isahaya wonders as well.
Kumagawa Incident Arc
At the end of the semester, Isahaya is present for Medaka's speech. She is surprised by Misogi Kumagawa's call for Medaka's impeachment. On August fifteenth, Isahaya is among the students who come to the Vice-President's Battle to cheer for Kuudou Hinokage. Isahaya is among the students who boo Class -13 after the Vice-President's Battle ends, stating they are no longer afraid of the Minuses. On August twenty-second, Isahaya and other students try to attend the President's Battle to cheer for Medaka, but are unable due to the nature of the match.
Unknown Shiranui Arc
At the end of the academic year, Isahaya is among the graduating students present at the Hakoniwa Academy graduation ceremony.
Bouquet Toss To The Future Arc
Isahaya is among the students who confront Medaka at the beginning of the 100 Flowers Run, holding flower number eight. She and the others are all blown away by Medaka's Kurokami Final. Isahaya thinks that she can now believe in herself, just as Medaka believed in her. All present wonder if Medaka thinks of their time together as good memories. Her message to Medaka is that Isahaya was glad to meet her.
Ten years later, Isahaya and Ariake are kindergarten teachers.
- Isahaya was voted the nineteenth most popular character in the series in the first popularity poll with 81 votes.