Poseidon

Poseidon is one of the three ancient weapons, alongside with Pluton and Uranus. It is a mermaid who has the power to communicate with Sea Kings.

Appearance
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 Nihong or "Mermaid Princess". Currently, Red has inherited Poseidon's power and her name.

Abilities and Powers
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 a 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. While 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's powers first awoke at age fourteen, she demonstrated no control over her own abilities and would inadvertently summon Sea Kings when she cried out loud, something which helped free her from the Celestial Dragons. Now, she has gained full control over her powers.

Original Poseidon
Poseidon lived during the Void Century and made a pact with Joy Boy. She would use her powers to command the Sea Kings to bring Noah to the surface and activate its intended mission, for which the details would be revealed when raised. However, Joy Boy failed in his commitment to Poseidon due to unknown reasons.

Current Poseidon
Red is the current poseidon, and often demonstrates her abilities to summon large sea kings to her advantage.

Trivia

 * In Greek mythology, Poseidon (Ποσειδών) is the god of the sea, water, and earthquakes. This notably keeps with the theme of her parents; Neptune is the Roman equivalent of Poseidon and Otohime is the daughter of the Japanese sea-god Ryūjin.