Princess Shirahoshi, also known as the Mermaid Princess, is a giant smelt-whiting mermaid and the youngest of King Neptune's children in the world of One Piece. Residing in the Ryugu Kingdom on Fish-Man Island, she is a shy and timid character but possesses a unique ability known as Poseidon, which allows her to communicate and control Sea Kings, massive sea creatures in the Grand Line. Her role in the series becomes crucial as her powers are key to the prophecy of the Ancient Weapons, making her an important ally to the Straw Hat Pirates.