This protector of Saint Lotier has lived in the nearby waters since antiquity and watched over its people. She once had fins to better dwell in the water but eventually chose to walk among humans, and so she obtained four legs via magic.
Official Description (High Mercury)
The Waterwyrm Mercury after releasing her true powers. Her immense mana acts as a great current that coats her body. With the kindness to protect those she cares for—and the strength to repel all threats—she's a truly wondrous dragon.