Hakofugu to Minamata no Umi  

The Boxfish and the Minamata Sea, ハコフグとみなまたの海

According to the book "Hakofugu no Negai" by Sakana-kun, it follows the journey of the main character, Hakofugu, who feels uncertain about himself. Along his way, he encounters Himetatsu, a lost child, and they embark together on their quest towards the "Sea of Minamata".