I'm convinced that the reason Kotori gets labeled a yandere is her School Idol Diary. There's a bit in the first chapter, where she's describing Honoka and Umi, and goes on about how Umi is so confident and attractive, how she gives the appearance of being invincible — and then just goes off:
"Well, compared to the word 'invincible', doesn’t 'rarely defeated' suit Umi better? Ah ah, if she knew that I had thoughts like that, she would surely kill me, but, Umi – I’m sorry. I can’t stop my imagination. Compared to a dashingly dressed Umi who clenches her fists and proclaims victory, a dejected Umi who is on her knees with both hands on the floor and tearfully admits 'I lost…' is so much more attractive, so much more outstanding, so much prettier –
"It makes my heart race so much faster♪....
"The Kotori who watches Umi’s archery competitions with thoughts like that is a very messed up person, isn’t she?
"Even if it’s only once, I want to experience what it feels like to be by Umi’s side when she bursts into tears...."
(Emphasis mine.)
She then mentions that she made a doll of "defeated Umi", which is "teary-eyed and dejected in defeat", and that it's her most precious possession.
Sooo. Draw your own conclusions. I don't think it's miles away from her portrayal in this book.
