NATSUKASHIKU OMOU KIMI WA DARE

Natsukashiku Omou Kimi wa Dare summary:

Ten years ago, he used to play a lot with a girl named Oto-chan. They promised to marry if he grew taller than her over the years. With modest but indelible memories in his heart, Hajime Aoima led a normal high school life - until the "promised day" arrived with Oto-chan.
Upon reaching the tree where they had made the promise, what Hajime sees before his eyes is an amazing spectacle.
A nostalgic and captivating suspense story full of charm and horror!