Biography
The Agent Scully is a sci-fi/fantasy character who insists that events can be interpreted according to mundane explanations. They never waver from this view, even though crazy things happen in episode after episode demonstrating how illogical or otherwise bizarre the universe is, prompting lectures to this effect — if they're not busy lecturing everyone else, that is. Once convinced that something is a Windmill, they will never step down from this belief no matter the evidence to the contrary, thus becoming a Windmill Crusader themself.
They may have no tolerance for flights of fancy whatsoever. If the character is a parent and their child merrily announces that they spent the afternoon playing with fairies, they may immediately retort "Fairies don't exist!" There will, of course, be little to no explanation given for why fairies don't exist; the fact of the matter (in their minds) is that they simply don't exist and you're being foolish for even giving the concept a moment's thought. The fact that children play pretend all the time and actually have a fairly firm grasp on what's real and what isn't is lost on them — such foolish thoughts must be squelched from their heads immediately! Likewise, they have no time for fairy tales — for these stories depict things that don't (or shouldn't) exist, which makes them nothing but frivolous poppycock, never mind the symbolic nature, moral lessons, and literary value they hold.
The same extends to any other magical or paranormal subject or fantasy of any kind — they have no time to think about or consider such things, and if you've been thinking about it you're an idiot who is wasting your time. End of story.
If magic or the supernatural actually does exist in their world and the character is aware of it, they may try to convince themselves it doesn't exist, or failing that, simply act as if it doesn't matter because respectable people don't go in for such foolishness. Scully Syndrome can ensue in extreme cases, where they are more willing to believe convoluted mundane explanations full of Plot Holes and swamp gas than the fact that they literally just watched an alien spacecraft land, had its pilot get out and introduce themselves, collect plant samples, and fly off again.
There is sometimes a sting in the tail, though, where the Agent Scully's disbelief actually prevents supernatural powers from working — in real life many alleged psychics blame "negative energy" from investigators for causing their abilities to fail, for instance. Quite simply, the Agent Scully does not clap, because they refuse to believe. This can be a powerful protective asset in a setting with dangerous supernatural elements, but should their disbelief ever falter, they can suddenly find themselves completely vulnerable.
The character is often an adult, but in some cases may be a child who is trying too hard to act mature, or how they think mature people act.
Derives its name, obviously, from Dana Scully, a character from The X-Files.note
An Agent Scully may also, obviously, be a Spock, and sometimes even a Straw Vulcan. Can be something of a strawman of those who currently doubt supernatural phenomena because of lack of evidence, placing them in a world where evidence of the supernatural is abundant and having them persist in their doubt. Truly egregious cases of this trope may even attempt to explain paranormal events with "scientific" explanations that are arguably just as much or even more paranormal than the events in question.
Compare Flat-Earth Atheist and Stupid Scientist. Contrast Agent Mulder.