The D, known in Japan as DD, was Aster Phoenix's legal guardian and the reigning champion of the Pro League for ten years.
In the dub, the name The D uses as Aster Phoenix's guardian is Kyle Jables. This is an allusion to the comedy band Tenacious D who sometimes call themselves The D. It's a pun of the band member's names, the band and their names Kyle Gass and Jack Black AKA Jables, which can further be suggested with the fact that his key card's Japanese name end with a D, being named Bloo D, much like the band, as well as being the band member's defining trait. His name in the Japanese version stands for "Destiny Duelist".
Sadistic and intelligent, The D serves as Aster's lifelong enemy due to the crimes he has to done to Aster's father. Although his actual appearance is short, through him, Aster was able to find out about The Light of Destruction and the cause for Sartorius' change in behavior.
When the D's normal personality is in control, his hair is smoothed back and he has an almost permanent worried expression on his face, however when his split personality takes over, his hair becomes wilder and spiky, his eyes narrow evilly and he has a sadistic evil grin. When The Light of Destruction completely takes over, his eyes glow red, as seen when he Duels Doctor Collector.
After learning about "Destiny Hero - Plasma" from Aster, who came to retrieve the card design after it was blown away by the wind, The D broke into Aster's family's apartment to steal the card. However, upon being interrupted by Aster's father, the D intrigued the interest of The Light of Destruction possessing the card, causing him to developed a sadistic and uncaring split personality bent on destruction. Thus enabled, he murdered Aster's father in cold blood (kidnapped in the dub, although he strongly alludes to killing him) and took the card for himself. After the theft, The D became Aster's legal guardian in hopes of avoiding detection. He later becomes the Pro League Champion and holds his title for 10 years. In unofficial underground Duels, however, he has taken the souls of countless Duelists with "Plasma".
The D then Duels Doctor Collectors in order to retain his title where he defeats Collector with "Plasma", setting the arena ablaze and subsequently doing away with Collector. He is then confronted by Aster, who is in total shock when he shamelessly confesses to his crimes. Although he uses "Plasma", it is trumped when Aster uses "Destiny Hero - Dark Angel", who was designed to counter "Plasma". After being defeated, The D gets caught in the explosion resulting from the card's purification. He swears to "return one day" before being engulfed in the flames, although his death is apparent in the Japanese version.