Rhea used to be a human girl who wanted to do something that would really benefit humanity. She volunteered to be the test subject for a new medicine, but what she didn't know was that she was really being tested on to become a weapon. The serum ended up working a little too well, and Rhea grew into a titanic monster and broke through confinement. Betrayed and angry, she lets out her rage on the world at large...
Original concept by Evannakita on DA and Twitter, interpretation by me.