Stalker was one of the seven members of a super-secret Stormwatch team created by Henry Bendix. The team consisted of Apollo and the Midnighter, the spectral Lamplight, the speedster Impetus, the sonic flier Crow Jane, and the super-strong woman Amaze.
When they took on their hero identities, Bendix erased all memory of their previous selves, so we know very little about Stalker except that he was a reptilian half-Daemonite hybrid who could turn invisible, and change his shape within the limits of conservation of mass. He appears to be the Wildstorm Universe's equivalent of Martian Manhunter.
When the team's first mission went wrong, Bendix disabled their transfer technology and let them die. Only Apollo and the Midnighter survived.
Years later, long after Bendix's death, the Authority and Stormwatch explored a secret bunker that Bendix had set up to clone himself into a super-powered body. The site was guarded by zombie clones of the first team, including Stalker, but the joint superhero teams put the zombie clones out of their misery.