Many, many, many, years ago...
When the Ocean Empire was fading away...the King of the Ocean blessed different particular Entities with the remaining of his powers. Those particular Entities were: a Whale, a Shark, a Squid, a Turtle, a Jellyfish, a Cod, a Salmon, an Octopus, an Orca, a Stingray, a Seal and a Dolphin.
The King's life force was connected to the Ocean, as it faded away...so did the King...
Those twelve Entities became known to all as the Council of the Ocean, they all had different Humanoid Forms none of them knew about. But the Dolphin was the first one to develop it.
KatherineOceansword, was chosen as the representative of the Dolphins. She was minding her own business when a group of rouge male Dolphins attacked her, they were mad that a GIRL Dolphin was chosen instead of one of them. Katherine fought with her life, just as she was getting overpowered, a Great Dragon called Grrr'ro're saved her. The Dolphins scrambled away as the Great Dragon roared at them.
Katherine was scared, she had never met a Great Dragon before, and when she was small her father told her that Great Dragons tried to kill the previous King of the Ocean. But Katherine knew there was something different about this one...she allowed the Great Dragon to bring her back to his Base and nurse her back health.
Katherine had imagined the dragon's Base as a huge dark cave, but she was totally wrong. The Great Dragon brought her not to a cave, but an entire city known as Atlantis. Grrr'ro're brought Katherine to the local doctor. The Doctor told Katherine everything about the Great Dragons.
Turns out they were not called Great Dragons at all, they were called Sea Dragons, but ones of royal blood were called Great Sea Dragons. The term Great Dragons refers to literally any Dragon with royal blood in them.
After Katherine was in full shape, the doctor brought her to Grrr'ro're to train. Grrr'ro're taught Katherine everything: combat, crafting, fishing, hunting, weaving, etc. The Sea Dragon helped Katherine unlocked her Humanoid Form, Sea Guardian. Sea Guardians were humans with ocean blue skin, but they could swim as fast as a normal Dolphin and have empowered abilities underwater.
When Katherine finished her training, she thanked Grrr'ro're and swam back to her kingdom