Moshi, Kono Sekai ni Kami-sama ga Iru to Suru Naraba.

Kurumi Haruka lives on the outskirts of Tokyo, raised without a father, keeping a quiet and unchanging routine—until a letter arrives. It was sent by her seven-year-old self. The words make no sense. She assumes a prank, until she discovers that four male classmates—Ace, Neji, Kyou, and Shuri—received identical letters. As strange incidents begin pulling each of them toward buried wounds and hidden pasts, something becomes clear: there is something fundamentally wrong with the world they're living in. The truth lies somewhere beneath a blue sea.