Ashita wo Kureta Kimi ni, Hikari no Love Letter wo

I don't know the identity of the person I've been pen pals with. A love story of tears and emotions! Magical i Land Award 2020 Novel Award Winner. Mizuki, who is unrequited love for her best friend's boyfriend, finds a love letter addressed to her in the "heart" that she picked up in her high school library one day. "I've always been interested in you, and I wanted to talk to you once." The sender's name is "Sato", but I have no idea. Who are you? Mizuki was suspicious at first, but as she continued to exchange letters, "Sato-kun" gradually became her cozy existence. What is the shocking fact that Mizuki, who is searching for the true identity of "Sato-kun", will soon find out?