Blue Mermaid

One day, Yoh meets Nana, a cheerful mermaid. They go to the movies together and he teaches her how to read and write, but then Nana says she wants to go to an event in town. The next day, Nana appears in front of Yoh with legs! Yoh enjoys spending time with Nana, but at the same time...

(Source: Hakusensha, translated)