Soryu: Soryu is the second-in-command of the Ice Dragons, a powerful mafia organization from Hong Kong that was originally led by his grandfather. His card is the King of Spades.
Baba: Baba is an infamous master thief. He is the oldest of the group and his card is the Joker.
Ota: Ota is a famous artist and dubbed the "Angelic Artist" by the media. He is the youngest and his card is the Queen of Diamonds.
Mamoru: Mamoru is the slacker detective. Eisuke bribed him into keeping the auctions a secret from the police. His card is the Jack of Clubs.
Shuichi: Shuichi is the ambassador for the Japanese government and is in good terms with many famous celebrities and other important figures. He appears to be sinister and sly, but is actually very kind. He appears in the second season and his card is the Ace of Diamonds.
Luke Forster: Luke is a British black market doctor with a collarbone fetish and knows Eisuke and Soryu from a prestigious boarding school in England. He appears in season two.
(excerpted from Voltage Inc Wikia) (pictures from Voltage Inc and google.com)