He is the Wildstorm tribute to occult detective John Constantine.
Very little is known about Jack Carter. During the 1980s, he was a major player in the occult underground of London. He trafficked in dark forces, talked with demons and ghosts and was one of the most powerful magicians in the world. Despite the darkness he walked in, however, Carter was an overall force for good.
Carter worked with the other costumed heroes of Britain, and together they tried to cope with the oppressive ruling government of the time. He also worked with Planetary, giving them insight into what was happening in England and Europe at the time. Some believed he was a savior, others a charlatan, but Carter did his work regardless.
Jack Carter apparently died in 2000, killed by a costumed metahuman whose life Carter had systematically demolished. Carter had faked his death, however, and with the help of Jakita Wagner was able to kill his would-be killer. Carter reappeared, his image radically changed (his new visage heavily resembling Spider Jerusalem), and then vanished into the night.