So everyone and their dad has seen the "Trudy is a robot" guesses, but I'm going to take it a step further and say ALL the characters are something more than they seem to be. Except they become these things by losing sanity points.

One of the things that can happen when characters grow insane in the Call of Cthulhu system is their back story can begin to be rewritten. So I suspect as the characters become exposed to the horrors of this world, they will begin to gain some of the power of these monsters and become a 50's style monster that represents themselves. Either that or in their insanity, they at least begin to believe they are monsters (or robot in Trudy's case).

My personal guesses are:

Francis is a werewolf because of the dichotomy of a nerdy loser becoming a werewolf.

Tony is a time traveler originally abducted by aliens , solely based on his use of the word "Rizz".

Kelsey is a 50 foot woman because I can't think of anything else.

And just to be contrarian, Trudy is the bride of Frankenstein, even though that isn't a 50s movie.