What's with the guy from Evil Dead?
Someone sees past the Masquerade, but either isn't shocked about it, or has gotten used to it.
A side character is kept in the dark as to whatever Government Conspiracy the heroes are involved in. Along the way, the side character encounters some strange occourences and events, but this is quickly dismissed as nothing by the heroes. Later however, the character will see or do something that exposes the Masquerade, such as witnessing (or more rarely killing) the Monster of the Week... ...and accepts it shrugs it off. Perhaps the character already knew or had figured it out on their own, or they may just have seen it all before. In any case, there is never any indication given that the character had prior knowledge of the Masquerade beyond rumors. Such characters usually only appear for a single episode. Repeated involvement by this character depends on where the heroes (and their superiors) sit on the Sliding Scale of Idealism vs Cynicism. While idealistic heroes may let the witness go, sign a non disclosure agreement, or even recruit them, Cynical heroes that only care about enforcing the Masquerade will have no hesitation about throwing them in prison, erasing their memories or Unperson identity, or outright murdering the witness. The trope namer is CIA Agent Burke from Stargate SG-1, who asks this after blowing up a Nicaraguan rebel zombified by alien technology.
Hello, Unknown Troper. You'll need to get known to lend a hand here.