Aaron Starkey

Aaron Starkey is one of several incarnations of Satan who appeared in the X-Files, he acts as the primary antagonist of the episode "All Souls" - like most depictions of Satan on the show he is a powerful demon who disguises himself as a human in order to corrupt the world and "All Souls" in particular was based strongly on the theological concept of "spiritual warfare".

History
Aaron Starkey was a disguise that Satan made in order to hunt down  four quadruplets who were physically deformed but in reality were nephilim, becoming a social worker Starkey manipulated Scully into helping him find the last girl, having failed to claim the others souls as a seraphim came and claimed them.

Ultimately Scully (who is a practicising Roman Catholic, despite her skepticism of paranomal phenomena) became aware of Starkey's true nature and surrendered the final girl to the seraphim - having lost his final chance to obtain any of the nephilim souls Starkey disappeared completely, Satan no doubt seeking a new disguise and plan by which to cause chaos or ruin.

Abilities
as Satan Starkey is a virtually immortal and powerful supernatural entity on a par with an archangel, however due to rebelling from God Satan is limited in multiple ways - as such he must rely on trickery and cunning to get what he desires : this is why he often takes the form of others and while some incarnations are shown to be capable of harming others in his guise as Starkey he didn't manifest any of his supernatural powers, he did betray his true nature at the end since as soon as he became aware that the seraphim had taken the last nephilim soul and thus denied him victory he vanished without a trace, having no further use for his "Starkey" guise.