Muromi-san

Muromi-san (波打際のむろみさん, Namiuchigiwa no Muromi-san) is a Japanese manga series written and illustrated by Keiji Najima. It was adapted into an anime television series by Tatsunoko Production in Japan from April 6 to June 29, 2013.

Plot
Takurō Mukōjima is an angler high school student who fishes out a mermaid named Muromi-san and develops rapport with both her and her friends. Muromi physically appears as a 16-year-old, but she and her sisters are ancient legendary creatures where they are both guardians of the Earth and much older than they appear. As Takuro who doesn't have a lot of motivation or care for all the attention Muromi grants him, the story follows him involved in Muromi's adventures, and the number of legendary monsters they meet.