Howard X Miller, known in Japan and nicknamed in the dub as X (エックス, Ekkusu) is a member of the Society of Light and a Pro League Duelist, ranked higher than Aster Phoenix (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.