The Sea King, ruler of the Sea Kingdom, fell into a deep slumber. Prior to his slumbering, he trusted the throne of the kingdom to his eldest daughter Uomihime. Jealous that her father chose her elder sister, Mikotsuhime, the Sea King's younger daughter, waged war against her sister. She created her own faction, called the Sea of Death, in an attempt to seize the throne by force. Many years went by as the Sea Kingdom fought the Sea of Death, until one of the Sea Kingdom's denizens, Meikai, sacrificed his life to seal away the kingdom's enemies. As well as sealing away the kingdom's enemies, Meikai's sacrifice formed the Cast Pearl, a pearl located above the Sea King castle, which is responsible for the protection of the Sea Kingdom.
- The Sea King
- Mikotsuhime (Formerly)
- Sal (Formerly)
- Old (Formely)
- Other unnamed Mermaid people
Sea Kingdom Gate
The Sea Kingdom Gate is the entrance to the Sea Kingdom. It is guarded by two crabs.
Deepsea Town (Umizoko)
The Deepsea Town is the hometown of Irena, Tarako, Aom, and Wadanohara.
The Coral Sea is a neighbourhood, and where the home of Cherryblod and Doloz is located. The Coral Path is a part of the Coral Sea.
Sea of Stars
The Sea of Stars is a place in the kingdom where stars fall.
Sea God Castle
The Sea God Castle is the home of Uomihime, and likely her father before her.