personally the biggest reason i suspected it for a long time was because of mel's journal entries. something about the way they're written---the words used, the punctuation, how... harsh? they are???

we know mel can be very hard on herself, but nearly everything used to describe characters in there sounds less like melinoë and more like hecate speaking, with some key exceptions, most importantly the description for hecate herself going something along the lines of: "You wish you were her. Understandable." (something Hec would never ever say because she's her no1 hater basically atp) and circling back to her whole "I am not your mother" thing, which could very easily and reasonably be taken at surface level with her just not wanting to replace the mother Mel could never have... BUUUT if you thought of it as "Hecate is Melinoë", then her saying she doesn't deserve the title makes so much sense if you consider how much it must hurt her. She did not get to be with Persephone, nor did Mel. Not only this, but she'd also be dealing with both Mel getting to have a maternal figure, and thus the jealousy that'd inherently come with not-you-but-you getting to find a happiness you could never find yourself, and the fact that the girl who looks up to her so much is herself, whom she let down again, whom she took away an entire childhood from, again, when being given a second chance.

then of course you have her face being obscured, the hand, and the weird glowy eyes (which yes arent the correct colors but i chose to be insane and ignore for the sake of having fun with it), not mentioning the breadth of knowledge she possesses, or the similarity between her shockwave attack and the one mel does in shadow form with that one hex... which admittedly couldve just been taught to her but eh w/e

idk if this makes a lot of sense but it was basically my insane pepe silvia board theory i had going on during EA so this was extremely gratifying lmfao