Arthur Grable

Arthur Grable is the secondary antagonist of the X-Files episode, Roland, he is one of the first of what would become a recurring theme in the series of vengeful victims returning from the grave to attack those that wronged them - despite this Arthur employed means that went far beyond simple revenge and thus earned himself a spot as one of the show's many "monster of the week" villains (this is a term for paranormal entities in the show not related to the over-all alien invasion "mythos" that the show was known for, ironically as time progressed the "monsters of the week" became far more popular and the alien invasion theme started to fade away).

History
Arthur Grable was a brilliant aerospace engineer working on an experimental jet engine design but tragically died in a horrific car accident before he could complete his work - thus his scheming colleague, Frank Nollette, stole his work and greedily accepted credit for the deceased genius' work.

However, Arthur had managed to have his brain cyrogenetically frozen and developed a psychic link with his mentally handicapped brother, Roland, who he possessed and forced to murder various workers in vengeance for having his work stolen.

Abilities
Arthur was a genius in life, far beyond most men in his field, after death he was able to cling to an ethereal existence via cryogenetically freezing his brain - this somehow allowed his spirit to mentally possess the body of his twin, Roland, through Roland he was able to return to a physical existence but was now a brutal and homicidal entity who had no qualms about forcing his brother to commit grisly acts of murder, seeking revenge for what he believed was stolen from him.