Sulfurina, or Ryu-san in Japanese, is the name of the embodiment of the Emerald Green Spring, a hot spring that emits a deadly poisonous gas consisting of hydrogen sulfide and sulfur dioxide. The group has to tread to retrieve sulfuric acid from the spring to make sulfanilamide to cure Ruri of her illness. Silver can be used to detect hydrogen sulfide to see where the poison gas is and to avoid it. She is not a real being but her illusionary representation has her looking like a beautiful goddess in long white dress, wavy, long emerald green hair with a flower crown on-top and big green eyes, white angel wings and wears thigh-high socks, but quickly turns into a gross, distorted melting face and even a giant skeleton demoness when people get too close and die from her gas. She is not a real being but has the appearance of a beautiful benevolent goddess that lures people in but once they get too close to the poisonous gas she emits, she reveals her true colors as a vicious demoness ready to swallow and murder anyone who gets within her clutches.