Foils are a narrative technique that goes back to Shakespeare where two characters are given antithetical traits so that their conflicts highlight their differences. Just to name two low-hanging fruit

I fucking love X-Files so much

although I love how after all the shit Scully goes through, her primary opinion even five seasons in is always "I don't know if it's a werewolf, Mulder. I'm sure it was just a coyote or something."

Man of Science, Man of Faith is easily one of the best episodes in LOST