X (エックス, Ekkusu (also known as Howard X Miller in the English dub) is a Pro Duelist hired by the Society of Light to duel Jaden, outranking Aster Phoenix in the Pro League (in the dub, he is Sartorius' lawyer, and claims he dreamed of being a pastry chef when he was a child). He successfully reduces the cards in Jaden Yuki's Deck to zero (twice), but ironically loses when he's unable to draw due to the effect of the "Neo-Spacian Glow Moss" that Jaden Summoned on X's field, causing him to Deck Out instead of Jaden.
In the English dub, his voice seems to be a parody/homage to Paul Lynde. His name is meant to be a pun in English, as mill (a term borrowed from Magic: The Gathering) denotes a direct disruption of a player's Deck. In the dub, he constantly makes court references such as "objection" when he activates a Trap Card.
His English last name, Miller, is a pun on his playing style, Mill.
As his name suggests, Howard runs a Mill Deck, which focuses on emptying his opponents' Decks, causing them to Deck Out and automatically lose. His strategy relies primarily on "Trap Sluzer" and "Monster Register", a combo that demolishes the Deck of his opponent, but leaves him virtually immune, while various Spell and Trap Cards are used to stall his opponents and protect this combo.