He is the main protagonist character in manga by Akai Higasa. Riddle is a young, elegant and mysterious man who introduces himself as "The Undertaker". He is tall, has long blonde hair, green eyes and beautiful face. He died when he was about 20 years old. Usually, Riddle wears a long black coat, top hat and boots with heels. Riddle is really a mysterious character. When half of Hayato's soul has been stolen he made a contract with him and gave him half of his own soul. He likes to tease with Hayato, treats him like his own toy. However, Riddle is always ready to rescue Hayato and seems to be jealous of him. When Riddle fights, he does a "funeral" for an evil spirits that eats souls. When he does it, he's saying: "I will play the funeral march that's right for you" and "My condolences".
He made a contract with Hayato because he possess the Twilight Key. Riddle wants him to become the King of the Catacombs and protected Hayato in secret since he was a little baby, long before they even met. He said "I couldn't let those spirits kill the future King before my very eyes.". When Hayato was a small child, not that many spirits tried to attack him and Riddle could deal with them alone. But when Hayato grew up, the Key's power had probably reached its peak, so Riddle came to Hayato on his seventeenth birthday and decided that if he couldn't protect him alone no more, it would be safer for Riddle to make a contract with Hayato and make sure that he learnt the skills to fight as an Undertaker.
Riddle got badly injured while protecting his contractor, but Hayato used the Twilight Key and heal him.