This agent has been seen appearing from wormhole/Stargate-like openings those who have spoken to him have heard identified as gyres, and then later disappearing into them again.
He sometimes helps GI JOE, he sometimes helps COBRA, he sometimes helps or thwarts them both. He claims to have seen a number of futures and has come to try to mold and shape events toward a specific, as-yet-unrevealed future.
Is he out of his mind? Does he have some sort of portable MASS Device tech he uses to teleport? Who is he, really? What is his true agenda? No one knows.
For a time, he is able to infiltrate GI JOE as Sightline and befriends a number of team members. During that time, he fights alongside GI JOE but the time comes when his purpose, still unknown, is fulfilled and he departs, and the Joes realize after the fact that Sightline was the Gyre-Viper all along.
Does he have a similar alter-ego in COBRA, MARS, etc.? No one knows...