Shadow is a robotic scientist who was created and designed by Dr. Tenma. After Dr. Tenma created Atlas, he realised that humans don't evolve as fast as robots, and were therefore limited in how quickly they could advance robotic technology. Out of this, Dr. Tenma created Shadow, a "robot that creates robots".
Shadow also creates these robots to fight Astro and force him to evolve until Astro became the most powerful robot in the whole entire world. Shadow wears hooded clothing so it's impossible to see his face, but at one point he takes his hood off revealing a robotic face with yellow eyes.
He is also the leader of Robotonia, when he removes his hood as shows Tenma's face.
In the final, he left Earth with Blue Knight and Robotonia in search of a peaceful world they could live in. Shadow was sure Dr. Tenma would go through any means to revive the damaged Astro, as Tenma is a mad man.
Shadow is exclusive to the '03 series