Ryugu Kingdom is the kingdom ruling over the Fish-Man Island. King Neptune is in charge of the Kingdom which covers the entire island of Fish-Man as well as its surroundings.
One Piece Episode 528 "Explosive Stimulation! Sanji's Life Crisis!"
Chapter 609 "Adventure on Fish-Man Island"
About 20 years ago, before the start of the series, Pirate King Gol D. Roger visited the Fish-Man Island in search of the Road Poneglyph kept somewhere in the Island. During the underwater expedition, Roger and Oden heard voices and were unaware that the local Sea Kings were their source. Roger reconnected with King Neptune once he arrived at the Island.
In the Sea Forest, next to Fish-Man Island and directly adjacent to the Poneglyph holding Joy Boy's letter of apology, was the Road Poneglyph, the location of which Roger had a suspicion. Roger encountered a young Shyarly while on Fish-Man Island, learned about several prophecies regarding the future, and expressed curiosity concerning Poseidon's whereabouts. After learning about the contents of the Poneglyphs, the Roger Pirates departed from the island.
A few years after Princess Shirahoshi’s birth, Queen Otohime (Neptune’s wife) attempted to bring about hope among the citizens of the Fish-Man Island to live peacefully alongside humans. She persistently worked on building ties between the World Government and the Ryugu Kingdom, an effort which was brutally shaken when she was assassinated by Hody Jones. Later, Jinbe inherited Otohime’s wishes and allied with the World Government and became one of the Seven Warlords of the Sea.
During the current timeline, the Straw Hat Pirates visited the Fish-Man Island after their 2 year separation. Soon, they were approached by King Neptune, who invited them for a feast in the Ryugu Palace. The situation soon turned upside down when Hody’s men attacked the palace and forced the Straw Hats into a battle to save the Ryugu Kingdom. The crew ultimately defeated the notorious gang.
The Ryugu Kingdom's buildings are primarily constructed to resemble seashells, and their foundations are coral reefs. There are several structures encased in bubbles floating in the air. Moreover, water streams arc through the air throughout the kingdom, allowing residents to swim within them for transportation. Additionally, using Bubbly Coral as an inner tube, humans can float in the air.
The royal residence of the Ryugu Kingdom is the Ryugu Palace, often known as the "Dragon Imperial Palace". It appears to be inside the smaller bubble sitting atop the larger bubble around the island.
The Hard-Shell Tower, on the northeastern side of the castle, is home to Princess Shirahoshi's quarters. It was formerly an arsenal.
A prison tower is located in the kingdom. It houses criminals like Vander Decken IX and members of the New Fish-Man Pirates.
The tale of the Mermaid Princess has been told throughout the realm. According to this myth, a mermaid can communicate with Sea Kings and is only born every few hundred years. Shirahoshi is the current generation's Mermaid Princess.
The Ryugu Kingdom owns the Tamatebako, a mysterious box that can age a person. At one point, Vander Decken IX intended to use it to age Shirahoshi so he could marry her by matching their ages.
Ryugu Kingdom is inhabited by the Fishmen and merfolks who rarely visit the sea surface. Among those are the main citizens of the Ryugu Kingdom that are mentioned below:
King Neptune is the ruler of the Ryugu Kingdom and a coelacanth merman. He is the current head of the Neptune Family, the spouse of the late Queen Otohime, and the father of Fukaboshi, Ryuboshi, Manboshi, and Shirahoshi.
Otohime was a goldfish mermaid and the queen of the Ryugu Kingdom, the wife of King Neptune, and the mother of Fukaboshi, Ryuboshi, Manboshi, and Shirahoshi.
The youngest of King Neptune's children and known as the Mermaid Princess, Princess Shirahoshi is a giant smelt-whiting mermaid and the princess of the Ryugu Kingdom.
Fukaboshi is the eldest son of the regal Neptune Family. He is a shark merman.
Ryuboshi is the second son of the regal Neptune Family. He is an oarfish merman.
Manbosh is the third son of the regal Neptune Family. He is an opah merman.
"King of the Sea" Jinbe is a great master of Fish-Man Karate and a whale shark fish-man. He is one of the Senior Officers of the Straw Hat Grand Fleet and the captain of the Straw Hat Pirates. He was a former member of the Seven Warlords of the Sea, and the captain of the Sun Pirates.
Madam Shyarly is a shortfin mako shark mermaid. She is Arlong's younger half-sister and the owner of the Mermaid Cafe. She used to be a fortune teller.
Camie is a kissing gourami mermaid. She works as a waitress at Fish-Man Island's Mermaid Cafe.
Hatchan is an octopus fish-man who, before the Arlong Pirates' defeat, served as an officer and a member of the Sun Pirates.
Pappag is a starfish and Camie's pet. He founded the Criminal clothing brand and teaches Camie how to become a fashion designer.
The late Fisher Tiger founded a pirate group called the Sun Pirates, also referred to as the Fish-Man Pirates. Eleven years ago, this crew broke up into three factions: the Sun Pirates led by Jinbe, the Arlong Pirates by Arlong, and the Macro Pirates by Macro.
When Jinbe was appointed a Warlord of the Sea, the World Government pardoned the Sun Pirates. However, when he abdicated his position, the pardon was revoked. After Jinbe rebelled and joined the Straw Hat Pirates, they eventually went to work for Emperor Big Mom.
Ryugu Kingdom is located on Fish-Man Island.
Queen Otohime is the wife of King Neptune of the Ryugu Kingdom in One Piece.
Princess Shirahoshi, also known as the Mermaid Princess, is the daughter and the youngest child of King Neptune.