Okay so basically one day Koyu was walking around the Japan when she stumbled upon the Yanaihara
The people in the Yanaihara, which included the teachers and Masachika
They took her in, despite not knowing her, k?
So when Ikumi and Shizuko invaded the school, and killed everyone, they found her and didn't kill her, don't ask why
Shizuko was like, "the fuck is this moon speak?" and Ikumi was like "speak no more"
So she studied across who knows where and found some information, she put the information into the diary
By this time Shizuko was almost done setting up, so Ikumi wrote the letter in the back of the diary and sent Koyu off to the Future Foundation, because fuck Shizuko