Poseidon

What the weapon takes shape as is a mermaid descended from the Ryugu Kingdom's ruling bloodline rather than any constructed device. Whenever a holder dies, the talent for conversing with the Sea Kings is handed down to those who come after her, and they take on her name as a title too, which is how the line called the Mermaid Princess came to be. Shirahoshi presently carries that name along with its awakened power.

At the heart of Poseidon lies the capacity to converse with the Sea Kings and bend them to one's will, something even the strongest merfolk cannot do. Since those leviathans can drown islands effortlessly, the gift counts among the most fearsome powers anywhere, capable of shielding countless lives when steered by love or wrecking the world when steered by spite. Among the Sea Kings, the Mermaid Princess holds the standing of a queen.

Shirahoshi could not rein the ability in when it first stirred at age six, calling the beasts forth by mistake whenever she wailed. Ten years on, during the fight with the New Fish-Man Pirates, the power reached its full bloom and she seemed at last to have it firmly in hand.

During the Void Century, the first Poseidon entered into an agreement with Joy Boy, promising to direct the Sea Kings in lifting the vessel Noah to the surface for an aim that would be made clear only after the task was finished. Joy Boy, for reasons never spelled out, broke his side of the arrangement. Once the Void Century ended she slipped out of recorded history, with the sole surviving evidence of her preserved in Skypiea on a poneglyph at Shandora and within the Sea Forest on the Ryugu stone, the latter reading as an apology Joy Boy had written to her.

Although it lay dormant, her power carried on through the Ryugu royal family generation after generation, while Fish-Man Island remained prepared to fulfill its part of the deal and the World Government outlawed reading the poneglyphs so the weapons would not stir. Across the years, a series of men bearing the Vander Decken name hunted in vain for the prophesied princess. Eight years ahead of the main story, a young Shirahoshi unknowingly drew Sea Kings to her when the World Noble Mjosgard menaced her mother, which tipped off Vander Decken IX, who set his mind on marrying her to seize her power and her beauty alike. Queen Otohime, knowing the gift might bless or curse them, drilled her three sons hard so they could guard their sister. Shirahoshi's ability fully awakened amid the clash with the New Fish-Man Pirates and ultimately rescued Fish-Man Island by stopping Noah, after which Caribou attempted but failed to seize her once he understood her nature.