Villains, particularly ones that look like one of the protagonists, are so often exposition for the “unrequited love” trope. And the beautiful thing is that it can work either way. Either Potential Lover #1 can tell that it isn’t Potential Lover #2, in which case we have confirmation of how well they know one another on a level that will accept no substitutes (as with Dean’s response to Cas’s first phone call) OR Potential Lover #1 cannot tell that it isn’t Potential Lover #2 and reacts in a revealing way to something that is out-of-character and often provocative (sadly haven’t had this here yet…I actually expected it from Casifer who did, at least, mock the way Dean and Cas say each other’s names with alternate gravity and desperation, like, even if they are going to the grocery store…but I digress).
Here, have an example of each from The X-Files, my first fandom and a mine of tropes. The first is from 4x20 “Small Potatoes” where a shapeshifter (yeahhhhh!) looking like Mulder attempts to seduce Scully only to be stopped by real!Mulder.
The mood in this is a little off; Scully can tell that something is weird about “Mulder” and has a line beforehand about how this is a “whole new side of [him]” but she’s not entirely objecting either. This gif doesn’t have Mulder’s reaction shot but he’s pretty surprised/nonplussed but what he sees. Her reaction is to freak the fuck out because, well, duh. Here’s a second example from 6x04 “Dreamland” where, quite honestly, I forget how this dude ended up in Mulder’s body. (It’s a comedy body swap plot like “Swap Meat.”) The dude ends up pretty convinced that Mulder and Scully are together (and this is the season after the movie and its almost-kiss) and tries to treat Scully like a sort of douchebag white guy would treat his girlfriend and gets this:
Yeah, no, you don’t talk like that to Scully (and Mulder would never call Scully “baby”…he won’t even call her “Dana” except if someone is dying). Partly, it’s a mark of their deepening relationship that Scully can tell it’s not Mulder in the first instance but not in the second and, partly, it’s because she reads one thing as far more out of character than the other: Mulder might try to kiss her but he would never treat her with that lack of respect.
This is already long but, just in case you need other examples, rewatch any post-S3 Buffy and keep an eye on things that look like her. The best example is in 4x16 “Who Are You” when evil!Slayer Faith swaps bodies with Buffy. The episode is a great take-down of the kind of rote morality that Buffy favors at this point in the narrative with, Faith-as-Buffy parroting “Because it’s wrong” to herself in the mirror as a way of learning her part.
The line returns later when Faith-as-Buffy goes out to the single bar in the entire town and encounters Spike (who at this point has a goverment-issued chip in his brain that keeps him from attacking people…yeah). At this point, Spike is an awesome character but they hadn’t yet figured out what to do with him as a series regular. This episode actually sets the stage for the relationship arc that will become one of the most important through-threads of the later series. While Buffy sees Spike still as an occasionally useful enemy, Faith-as-Buffy can tell immediately that he’s attracted to her and exploits it to taunt him:
( “And you know why I don’t?” [Spike’s face says “why the hell not?”])
So, Spike is all WTF-turned-on-but-furious-about-it but can’t quite tell at this point that it’s not Buffy. Once again, the villain-looking-like-a-protagonist makes subtextual feelings textually obvious both to at least one character (Spike, in this case) and to the audience as a way to prepare them for an in-text relationship that does happen eventually.
So, that was a lot about two shows that are not SPN
but which maybe you guys like too but both lean heavy on the relationship tropes and both have supernatural elements to them too. (I didn’t even talk about the Buffybot…of all the things SPN has done at least no one has yet had a robot double.) My speculation for S13 is that it will be borrowing from the same playbook. Maybe Asmodeus-as-Cas will appear to Dean at some point to “prove” that Cas is ok (omg stop with the phone calls, worried husband Dean!) and say something OOC cruel to imply that Cas is aware of his feelings but doesn’t requite them. This would be similar to The Empty voicing Cas’s worst fears because it knows that’s where he’s vulnerable, perhaps spinning him out into the “adultery” that Tink predicts. It would make the full misunderstanding textual and perhaps eventually force real!Cas to refute it. Maybe Asmodeus-as-Cas makes the same mistake as body-swap-Mulder and talks to Dean like a lover so that Dean, like Scully, can confront him about obviously not being real!Cas (like we got with Cas’s code for “SOS TROUBLE” being “I miss you baby”). I seriously doubt we could get anything as juicy as the Buffy example so save it for fanfic.
Tl; dr: SPN is using the same trope playbook as other popular genre shows and will probably feature a villain disguised as a potential lover to reveal and make textual (to us and maybe to Dean or Cas) the feelings that the lover fears are unrequited. We’ve already had this once, let’s have it again with Dean watching!