Dexter

Dexter, known as Tenpei Hayano in the Japanese verion, is a friend of Leo. Like Leo, he is a duelist.

Biography
He and Leo went to the Daimon Area to duel the Black Rose. Dexter had modified his Deck specifically to duel against her. When he first meets Yusei, he was afraid of him due to his Criminal mark, but began to admire him once Leo told him he had customized his and Luna's Duel Disks to fit them and was strong enough to enter the Fortune Cup.

World Turbo Grand Prix
According to the third opening, it appears that he has enlisted into The New Domino City Duel Academy along with Leo, Luna and Akiza.

Deck
His deck is unknown, but he had once claimed to have built an anti-plant deck to counter Akiza, though he had never dueled onscreen before.

So far the only shown card from his deck is Batteryman AA, so it is probable that he uses either a Batterymen Deck or a Thunder Deck.

In Yu-Gi-Oh! 5D's World Championship 2010: Reverse of Arcadia, he uses a Fairy Deck titled "With Intelligence".