I'm pretty sure Fou fits in there somewhere. The fluffy murder beast is kind of a subtle Big Deal before the end of Part 1.
Nobody, and I mean nobody, saw it coming as to whatever Fou was. With the sole exceptions of da Vinci and Yu Mei Ren.
da Vinci, being the genius she is, knew something was up with Fou, even prior to the various divine spirits having an 'Oh Fuck Me...' reaction upon seeing him (which isn't until muuuuuch later on). But couldn't prove it without potentially risking Chaldea, so kept her mouth shut. Afterwards, once the Grand Order started, she had even greater reasons NOT to reveal things (says something that even Holmes has kept his big mouth shut), as Fou, potentially, would have been just as massive of a threat - and had access to the INTERIOR of Chaldea - as Goetia. Fou is, after all, the ultimate predator of Man. However, she has
never managed to confirm her hunches.
Yu Mei Ren, OTOH, does know what Fou is from the very start,
but doesn't care. They're two sides of the same coin after all, and she never was human to begin with, so doesn't fall under his jurisdiction (technically, but then again, she's concealing herself as a human, which gives him an 'in'). Neither had an interest in bothering the other, so they simply....didn't interact. He wasn't going to interfere in her attempts to summon her beloved, and as long as he wasn't going to, she had no reason to even remotely bother him as to whatever he was doing/planning.
Beyond that, Fou's true status is utterly unknown to everyone else in Chaldea. Which is hilarious in hindsight, as Beryl should have been reacting like crazy to Fou, yet wasn't. Fou is
that good of an ambush predator.
As it stands, the only ones on the Shadow Border who know the truth to Fou: are the Divine Servants - whom Fou gave death glares to - which promptly STFU'd them on the spot; King Hassan, Super Orion, and Romulus-Quirinus - who know because they are meant to defend against Fou/others like Fou - yet have
very good reasons not to piss off/awaken the floof; Kiara, B.B., and Mara - because of what they
were, and
possibly Holmes or Dantes. Holmes, because he really is a genius, while Dantes is nearly as good, and has ways to 'cheat' in learning something's background. Everyone else, Mash and Chaldea's master included, don't know. Part of that is Fou being generally sneaky in nature, the other is that Fou deliberately wiped the truth of how Mash survived, from the memory of Chaldea's master.
Fou's reveal, was almost literally THE best kept secret in the entire game, bar none, up to that point. There were hints as to him being something
slightly more, and were vague enough to never really tell you anything, but nothing hinted towards him being.....
THAT. There's a
reason the JPN servers, 2chan, reddit, and various other gaming sites, went absolutely 105% apeshit crazy the day you could reach the Season 1 epilogue happened, and it wasn't the Goetia fight, nor the Solomon reveal.
It was the floofball.
I imagine Taylor, if she
ever finds out that is, will be having panic attacks for ages afterwards. For Good Reason.
"I touched Fou. I
petted Fou. Oh my god, I
fed Fou."
]Shaking & Shivering Intensifies[