Hagure Ningyo no Diana

A certain country long ago. A mermaid was brought to the castle.
The rare mermaid with transformed legs, she was supposed to be presented to the king of a neighboring country...
But the prince who saw the mermaid said, "Would you like to be my princess?"
A communal life that starts from there! How will the love of these two people go...!?

(Source: Urasunday, translated)