Mermaid.

When Do-Hui, an orphaned girl, is searching for her missing mother, she meets Si-Hyun, a merman. Si-Hyun promises to help Do-Hui find her mother, and in return, Do-Hui needs to help Si-Hyun find her true love to become fully human. However, strange things begin to happen around Do-Hui. Is her mother's disappearance related to Si-Hyun's appearance? (Source: WEBTOON, translated)