Now currently I’m dealing with the “character in a blatantly fantastical setting is a staunch realist who doesn’t believe in supernatural” trope, but I’m at least trying to give it a good explanation.
This is someone who is used to seeing magic and wraiths as normal, but doesn’t really have much concept of the really weird beings in his world. People can use magic? Cool, it’s all elemental stuff that has at least some basis in science, at least insofar as “this makes sense, it has rules, it has boundaries.”
Weird monsters of no known origin wander around a lot? There’s a few proposed theories (aliens, cosmic force affected by human emotions, ‘normal’ organism we have yet to fully understand) and plenty of people live in a world where not everything is fully explained or makes sense to them but they still see it and accept it as “normal” and not “spiritual.”
The Jinn? In-universe, those are considered a conspiracy theory on par with Bigfoot. Aiden, who is obsessed with the Marvelous Ones/Jinn/Grigori, sounds every bit like those “Bigfoot is real and he sold me weed behind a 7/11″ memes when she claims she was friends with a Jinn. That’s why people see her as weird.
So yeah, Griffin is definitely one of “those” people, who only believes in “rational” things even though he’s a mage who can control air and lightning, because he only believes in what makes sense to him.