Neptune

King Neptune (ネプチューン王) is the king of the Ryugu Kingdom on Fishman Island. He has four children (Fukaboshi, Ryuboshi, Manboshi, and the Mermaid Princess).

Appearance
He has yet to make an appearance. However, it can be assumed that he is a Merman.

Personality
He appears to be very protective of his family, as he had his sons form an army solely to track down Van der Decken IX so that the pirate would stop harassing his daughter.

Abilities and Powers
As king, he has royal authority over the entire Ryugu Kingdom. Whether he has any fighting ability is unknown.

While he is the king of Fishman Island, he has had to rely on the protection of two of the Yonkou: Whitebeard until his death and then Charlotte Linlin (Big Mam) afterwards. He didn't have to pay collateral to Whitebeard, but he does have to pay a monthly fee of sweet candy to Big Mam.

History
He was so angered by Van der Decken IX pestering his daughter through letters, packages, and ultimately marriage proposals, that he had his three sons form an army called the Neptune Army to capture him.

Trivia

 * His name comes from Neptune, the Roman god of the sea.