Styx

Styx (スティックス, Sutikkusu), otherwise known as Edgar (エドガー, Edogā) by his host, Ellen Poe, is a Demon wandering Earthland, with the help of a human girl, due to his natural weakness to light, helping her live by replacing her wounded heart, while she escorts him to his destination, sharing somewhat of a symbiotic relationship.

Trivia

 * In Greek Mythology, Styx is a river that formed the boundary between Earth and the Underworld.
 * Styx's nickname, Edgar, is a reference to Edgar Allen Poe], a famous American author, who is famous for his gothic horror.