Girls of the Thirteenth Month

Kazuho Anamori, a high school girl, was unable to fit in with her class because she was reluctant to withdraw. One day, by chance, she met Miko Karakida, who was born with heart disease, and they became closer. However, six months later, the shrine maiden died while in the hospital. A letter arrives at Ichiho who is sad.
“I have a story about Miko Karakida. Come to that room tomorrow night.”
What is the story about the shrine maiden? Ichiho goes into the school building at night.
A mysterious transfer student, Rin Ukima, who appeared at the same time as the death of the shrine maiden, was waiting there.