Aria's father was a human and her mother a mermaid. Humans had just began to turn hostile towards merpeople because of the allure of wealth, and the knowledge the humans had of their abilities to lure others to their deaths. Merpeople had began to return hostility in hopes to prevent any more of their kind from being fished and killed for their scales and artifacts. This couple hid away and interacted in secret, away from their races to avoid being labeled as a traitor. Aria's mother did not want Aria at the time she was known to be pregnant, and when she was born her mother and human father tossed her into the sea to rid evidence of their relationship. She was found by an old mermaid and raised. There was a period of peace between the races for about 10 years, but then history repeated itself. Aria saw a bunch of merpeople gathered above the water and went up to the shore to take a look. She was caught up in a battle of sorts; I wouldn't call it a war. In the process, many merpeople perished and when they discovered Aria was there, they hid her on land so that she would not be harmed. She went through a panic attack while she was hiding and ended up passing out.
She remembers none of that, and the latest she remembers is waking up on the sand (with legs) and being very lost and confused. She does have triggers that cause her to remember flashes of things that happened before she passed out, however she has no permanent recollection, and the panic attacks still happen when she is confronted with the feeling of immense fear.
She met a man on the shore who covered her up with one of his old blankets and gave her some food, but was unable to stay and take care of her. She turned to the ocean as it drew her in, and she returned to the ocean, a bit surprised about the tail but it felt so natural to swim and live in the water that she assumed that was where she belonged. And now it is about 7 years later, where her life really begins.