The Getter Emperor was an entity from an alternate world seen near the end of the OVA.
The Getter Emperor is gigantic machine the size of a planet resembling the Getter 1. It resembles its original counterpart with a few minor differences including the number of fins on its head, the position of green panels, and compared to its counterpart, appears more UFO-like much like the manga version's components.
The Getter Emperor is an immensely powerful being able to use Getter Rays to levels beyond comprehension. Its Getter Beam is able to destroy an entire fleet of Invaders. Its entire energy output is even stated to surpass the big bang. Its so powerful that it can open up portals between realities for its hands to fit through. Surrounding the Emperor constantly are smaller ships that resemble its components that it commands. It also seems capable of creating illusionary figures based on what the Getter Rays accept including Ryoma Nagare. All of this is possible through a self-evolutionary function that happens with every battle.
After defeating Cowen and Stinger through the Shin Shine Spark, the Getter Teams are confronted by a projection of Ryoma in a space-time crevice saying that he was waiting for them. The Getter Emperor is then seen in sight with its hand reaching out to them. The older Getter Team manages to get their younger counterparts away while they are dragged off into an alternate world where numerous Getter robots are in constant combat.