Ragradia in the anime.

Aqualord Ragradia (Japanese: 水竜王アクアロード ラグラディア Romaji: akuarōdo raguradia and known as the Water Dragon King in the English dub) is one of the four Dragon Kings left by Flare Dragon Ceifeed before his death after the last battle against the ma-ō Ruby Eye Shabranigdu. Like his three siblings, Ragradia possesses less than quarter of the original power of Ceifeed. Aqualord protects the northern zone of the Slayers world.

Concept art of Ragradia.

When a fragment of the Demon King which was resurrected from within the sorcerer Lei Magnus teamed up with Chaos Dragon Garv during the Kōma War, they attacked the Aqualord, who was by then isolated from his ryūzoku troops and fellow shinzoku. He managed to seal Garv into a human body and imprison Lei Magnus-Shabranigdu in a block of magical ice. However, Ragradia died as a result of using up all his power. A part of his will would go on to exist in the Claire Bible, which contains his memories and knowledge. The Eternal Queen also has similar abilities, and so does Auntie Aqua, who appeared in Slayers NEXT. The latter only appears near complete copies of the Claire Bible.