Jamgyeojugeodo

Hye-Seong, a boy living on a remote island, and Yun-Hae, a mysterious mermaid, become each other’s only friend. But soon after they meet, they’re separated after Hye-Seong moves away. Six years later, Yun-Hae develops a mysterious power and finds Hye-Seong again. Unfortunately, too much has changed, and Hye-Seong can no longer dedicate all his time to her as he did in their childhood. He tells her they are too different, but Yun-Hae’s attachment to Hye-Seong slowly transforms into an obsession...

(Source: WEBTOONS, edited)