Shiromaru

A white, round SOMETHING has appeared in the Sea of Japan! The notoriously lazy Harusame gets appointed as the mysterious marine creature "Shiromaru" handler along with two quirky companions! While Harusame wants nothing more than to avoid human interaction, his encounters with Shiromaru force him to confront the past... A coming-of-age sci-fi drama for anyone who finds this world hard to live in!

(Source: MANGA Plus)