Ashita wo Kureta Kimi ni, Hikari no Love Letter wo

Mizuki, who has an unrequited crush on her best friend's boyfriend, one day picks up a love letter addressed to herself in "Kokoro" in her high school library. - find "I've been wondering about you for a long time, and I wanted to talk to you even if it's just once." The sender's name is "Sato", but I have no idea. you Who the hell is that――? At first, Mizuki was suspicious, but as they continued to exchange letters, "Sato-kun" gradually changed into a comfortable existence. What is the shocking fact that Mizuki, who is searching for the true identity of "Sato-kun," eventually learns? https://www.kadokawa.co.jp/product/322205000459/