Stel is a member of the galaxy-spanning police force called the Green Lantern Corps. As a living robot, Stel offers a unique perspective to the Corps, as well as the ability to rebuild and upgrade his body as necessary.
For the continuity-minded out there, Stel is based on his appearance from Green Lantern vol. 2 #183, when he was resurrected from death to save his planet from invaders. It features the most detailed art from Carmine Infantino I've ever seen - it has a George Perez-level of detail.