The Betrayed

There was once a lonely werewolf, his name is ōkami no ō which means Wolf king, he is the captain of a pirate ship.
He's sailing close to an island they started calling Mer-island.
Convenient name?
No, the day they started calling it that, they saw a beautiful mermaid.
She looked about 6 years old, she had long, light brown hair.
They started to call her Umi no utsukushi-sa, which means, ocean beauty.
They saw her almost every day, she always looked so sad.
They saw her changes over the years, one thing did not change, she always looked sad.