Poseidon

Poseidon is one of the three ancient weapons, alongside with pluton and uranus. It is actually a mermaid who has the unusual power to communicate with and command the sea kings. The power is currently manifested in shirahoshi, the 16-year-old mermaid princess of the ryugu kingdom.Rather than being an inanimate object like usual weapons, poseidon was a mermaid belonging to the ryugu kingdom's royal bloodline. After she died, her descendants who possessed the same ability to speak with sea kings were granted her name as a title, known colloquially as the "Mermaid princess" (人魚姫, ningyo hime?). Currently, shirahoshi has inherited poseidon's power and her name.

Poseidon has the power to communicate with and control sea kings, an ability that not even the most powerful merfolk possess. This is considered to be an extremely powerful weapon because of the sea kings' destructive potential to sink islands with ease, and to control such a force is regarded as one of the most fearsome abilities in the entire world. When guided by the power of love, it can save thousands of lives, but with evil intentions, it can sink everything into the ocean. The sea kings refer to the mermaid princess as their queen.When the current mermaid princess shirahoshi's powers first awoke at age six, she demonstrated no control over her own abilities and would inadvertently summon sea kings when she cried out loud, something which could put her kingdom at risk. Ten years later, during the battle against the new fish-man pirates, she fully awakened her ability and seemingly gained control of it.

Eight years before the start of the series, princess shirahoshi had unwittingly demonstrated the ability to summon sea kings when her mother was in danger of being shot by the world noble mjosgard. Her cries summoned a group of the sea monsters, allowing her mother and vander decken ix to find out about her ability. At this point, decken deduced that shirahoshi is the mermaid princess of legend that his family had been searching for and decided to marry her to gain access to her fearsome abilities (as well as her unparalleled beauty), leading him to send her threatening marriage proposals for the next ten years. Otohime, knowing that such powers can be both beneficial and deadly, had her three sons train hard to become strong warriors in order to protect shirahoshi when the time came. Shirahoshi's power was fully awakened during the fight with the new fish-man pirates, which neptune noted was not a good thing. However, it led to the sea kings stopping noah from falling on fish-man island. Indeed, when caribou overheard nico robin and king neptune discussing about ancient weapons and shirahoshi as poseidon, he swiftly tried to kidnap her for her apocalyptic power, only to be thwarted by the straw hat pirates.