He currently owns the soul of Willard Wyler, and is the master apparently of anyone who defeats Wyler, or later, the four actors.
After defeating Mephistopheles, it is revealed that the four actors cannot escape due to the fact that they are stuck in a Soul Jar at the original theater, meaning that the theater from The Beast from Beyond is simply based on the memories of the four, and that they can never escape. Whether the Soul Jar belongs to Wyler or Mephistopheles is unknown.
Mephistopheles has red skin, with glowing demonic symbols, three on his chest, and one on each shoulder. He also appears to have tentacle-like wings with the words, "Kneel before me and despair" written in symbols. He also has a six pack and wears a brown undergarment. His eyes and mouth apeear to be glowing in a similar fashion to his symbols, and he is surrounded by an aura of what appears to be fire. Just like a stereotypical devil, he has horns. He also is capable of changing his shape at will, as demonstrated by his appearance as a humanoid figure with curly hair, similar in size and shape to Lee Ross and as Pam Grier to show the four actors that they are only in a manifestation made by him of the theater.