Jack Williams or "Black Jack" is a mutated human located in Rashia who is the leader of Black Jack's Bunny Brigade, along with Clover, Cotton, and his brother Rai.
Black Jack is a mob boss of an organization called "black Jack's bunny Brigade" in an alternate world that connects to the Other World. He apparantly struck up a deal with Skye to give him a mysterious red power that enabled him to work his way to the top and command a legion of fellow bunny brigadiers. Jack and Skye's relationship is a strained partnership that often comes off as a playful rivalry of who has the better organization and minions.
Jack (on the left) and Rai (on the right)