Polaris on Azure

Yui, who lives in a small fishing town, has a childhood friend named Kujira, a mermaid. When Yui was a child, he found Kujira wandering alone in the sea and invited him to his house, and they have been friends ever since. Since then, Kujira has lived on land with Yui without returning to the sea, but his voice has a special frequency and he is unable to speak to anyone other than Yui. Yui, who is now a high school student, has a faint crush on the grown-up Kujira, but is torn between wanting Kujira to be dependent on him and wanting Kujira to see the wider world. On the other hand, Kujira also begins to have a certain desire for Yui, perhaps because of his strong feelings for him…?

(Source: Kaiohsha, translated)

Note: Includes one extra chapter.