Kotonoha just wanted to make Mokoto happy just so he would stay with her, that's why she started getting sexual..
In a way, that DOES make her a slut, but not really in my view.
One example of the yanderekko is Kotonoha Katsura from the adult visual novel School Days. She becomes Makoto Itou's girlfriend early on in the story, but Makoto is later stolen from her by Sekai Saionji, Makoto's friend who had initially been their matchmaker. One of the bad endings in the game involves Kotonoha murdering Sekai by slicing open her throat with a Japanese saw in broad daylight, and another has Kotonoha throwing herself from a ledge and dying in front of Makoto and Sekai, traumatizing them forever. In the anime version of School Days, the stealing of Makoto is followed by lots of misfortune that eventually makes her break down. Sekai kills Makoto and Kotonoha kills Sekai; she then escapes in her family's yacht, carrying Makoto's severed head with her.
best ending was Kotonoha's ending on christmas eve <3
that was THE BEST, THE BESTEST!