What character is played by actor Nicholas Lea in The X-Files?

The character played by actor Nicholas Lea in The X-Files is Alex Krycek.

Krycek is introduced in the second season as an FBI agent assigned to work with Fox Mulder. He initially appears to be a trustworthy partner, but the series soon reveals that he is willing to cooperate with multiple factions, including the Syndicate and alien interests, whenever it serves his own survival.

Unlike a straightforward villain, Krycek repeatedly changes alliances. He becomes one of Mulder and Scully's most persistent adversaries and is especially connected to Assistant Director Walter Skinner, whom he manipulates and even infects with a nanotechnology-based weapon. His shifting loyalties earned him the fan nickname “Ratboy.”

Nicholas Lea had previously appeared briefly in the first-season episode “Gender Bender” as Michael. His performance impressed the producers, leading to his much larger role as Krycek. Jeffrey Spender, the Cigarette-Smoking Man, and John Doggett are separate characters.