Ocean, formerly known as Sedna, is one of the gods of the realm. She is in charge of the oceans of Farir, and in Arc 2 consumed the Star of <to be edited>.
Before 4Kingdoms (Arc 1)
Sedna was one of the three children of Earth and End. When Sedna was born alongside her siblings Desdemona and Calvin, she was unfortunately stillborn. End stated that he saw her soul departing the realm as she died, but he was not aware that it was her soul. Earth concealed the existence of Sedna from End, and he was not aware that Earth had had triplets rather than twins until many, many years later. They determined that Sedna would have grown up to be the goddess Ocean, had she lived.
Before 4Kingdoms (Arc 2)
In this timeline, Sedna survived her birth, and grew up alongside Desdemona and Calvin under the care of Cecilia and Tobias. She became an adult, and eventually consumed a star and become a goddess like her siblings and parents, adopting the name of Ocean.
She built large temples underwater, places for her friends to live, underwater creatures called guardians. She spent a lot of time with them, swimming around.
Eventually, she and Earth were murdered by End, and their deaths led to End's imprisonment in Lunestra.