So I've been thinking about them:
Specifically I was wondering what the moment was (if there even was a specific moment) that cinched it for Twilight developing feelings for Yor.
[Spoiler warning: this post references manga chapters not yet animated]
I think for Yor it's pretty quick. Like, this moment here:
Not that Yor fell in love with Twilight then (ymmv) or that she's fully aware of her feelings, but it's explicit that she felt connected to him here and attached in meaningful ways.
But for Twilight, it wasn't so clear. For a while I'd kind of decided that it just came over him slowly (and I think there is something to that) and that there wasn't any singular moment which stood out. But that didn't feel quite right. The more I thought about it, the more I thought there were two stand-out moments, only one of which Twilight actually (semi-)clocks.
The first, which I think passes him by entirely, is this:
In my view, this laugh is an entirely authentic response. I think he is, despite himself, delighted by this woman who 1. just unexpectedly saved him from being stabbed, and 2. did it by sending the guy flying across an entire alleyway.
This is accentuated in the anime, I think, by the jaunty, puckish music that makes up the first part of their marriage theme song. I am dying for the reappearance of this music in some fashion, btw, it's so fun and cheeky and I'm hoping foreshadows their vibe after various revelations and particularly when they start working together as Agent Twilight and Thorn Princess:
The second moment for Twilight, I think, is more subtle for all it's more impactful. Or at least, the degree of its importance passed me by on initial read/watch, and I think it's deliberately downplayed by Twilight himself. Because he does actually clock it but if he looks more closely at it, well... then he might have to do something about it. And maybe that something won't comport with what the mission needs, and then what?
It happens when Twilight first bugs Yor, and then poses with Franky as SSS agents to test whether she knows Yuri is with the SSS.
It's clear in the lead up that Twilight recognises he has some feelings about/for Yor, and he doesn't want to spy on her; he doesn't want to mistrust her at all. He has to convince himself to take seriously that she may be a potential threat.
And even then, the convincing only sort of mostly works, because he hesitates again:
Which is, by the way, bananas. At this point, they've been a fake family for maybe a handful of weeks? Twilight is an experienced, accomplished spy with a finely honed and necessary sense of paranoia. Of course he should be suspicious. Her brother is an SSS agent! Canonically, the SSS are both Twilight- and SSS self-described as Twilight's greatest existential threat. It shouldn't be a question whether or not to verify Yor's knowledge here. And yet.
We all know how the rest plays out. He decides that listening in isn't enough, he needs to confront her insofar as he's able. I wrote previously about Twilight's relationship with Anya and the pivotal moment for him in how his view of his relationship with Anya changes based on Anya's (and Endo's) choices. I think a similar thing happens in this scene with Yor.
See, it would have been enough for Yor to continue to deny, continue to not call on Yuri's help, to prove she didn't know, and to put Twilight's mind at ease.
Endo takes it further.
Y'all: THIS IS ABSOLUTELY WILD. It borders on levels of impulsive foolhardiness that Twilight should actually take as a negative for the person playing his wife for Operation Strix. Yor even alludes later to the problems this could cause!
The SSS are indiscriminate; if Yor was facing down actual SSS agents, first assaulting and then threatening them would 100000% land her in custody. Were it not for Yuri, it may even get her disappeared, based on how casually and frequently Yuri references having people executed. It would absolutely put the Forgers at risk, in general and in the implicitly sexist Ostanian society, because if Mrs Forger behaves this way, how does Mr Forger behave? And why can't he control his wife? The Secret Police are not known for their leniency, their modesty, their discerning, their temperateness, their mercy. They are known for the exact opposite of those things. And due to being a spy, Twilight probably knows they're actually much worse than even their public reputation.
And here's Yor saying: you can question me but if you threaten my brother or my husband, I will fucking end you. Bodily.
Of course, it's entirely in keeping with her character, and it's an entirely revealing moment of who she is. And I think this is the moment for Twilight. He's already been trusting her bit by bit, as he says above, intuitively. I'd suggest that maybe even more than that though, Yor taps into something Twilight deeply wants: backup. Someone and somewhere safe. Maybe we could describe a person fulfilling that role in an adult relationship as a partner...?
It's because he doubts his intuition (his wants, his feelings, things he shouldn't be countenancing) that we get to this point where he (overzealously) tests her.
She blows his test right out of the water.
The SSS are basically the group he fears most; this is reiterated throughout the story. He doesn't trust them specifically because of who he is and also just generally. He doesn't trust their judgment. He doesn't share their values or their priorities. He doesn't like them around. He doesn't like them looking. He doesn't like being anywhere near them. (Also, he's right.)
And here's Yor. Not only standing up to them on his behalf but actually going on active defence on his behalf.
(I pause here to note 'on his behalf' is a bit, mm, tricky, since it's actually technically on Loid's behalf and I have Thoughts and Feelings about Twilight & Identity. But for the sake of the impact of this moment on Twilight, we'll take it as writ that in this moment there's no appreciable difference between Twilight and Loid.)
I think from here on out, it's incredibly difficult for Twilight to ever doubt or distrust Yor. He perceives her as firmly in his corner, that if the chips are down — if his worst enemy and his worst fear come knocking — she'll be on his team, unflinchingly. He may not think there will be much she can do (heh.) or much she can offer given the power of the SSS and her civilian status (I reiterate: heh.), but it matters that he believes that she'll be by his side.
And you know what? He's right. She will be.
That isn't something he's had since he was a little boy. Even WISE doesn't seem to offer that to its agents, given Nightfall's thought here:
Twilight's had to rely on himself for decades and now here's this astonishing woman who will threaten the Secret Police for his sake. Of course he trusts Yor. Of course this moment widens the cracks in his barriers. And further: of course those cracks start to reach into those walls deep, deep inside that protect his heart. This is all before getting to other moments, like when he reflects on how Yor is creating a better world in ways he (thinks he) can never aspire to do himself. That she loves Anya openly, freely, with such dedication, to the point of sacrificing her own needs. That she just never gives up, she persists and persists and persists, always doing her best. That she reminds him it's okay to accept peace and to rest. That she wants and tries to take care of him... On and on and on.
Of course we get to this point:
I'm particularly taken with his body language a little later in the scene. He manages to get himself to sitting but he's still sprawled, open, even as he can't wrap his mind around what exactly is happening or why, and he's feeling vulnerable for all that. But at the same time, this is Yor. And she's safe.
In my view, if the Mole Arc hadn't happened immediately between this moment and the earlier where Yor declares herself unhappy, it would have been clearer how much stress he felt specifically due to Yor's apparent sudden unhappiness with their arrangement. The stress got subsumed (conveniently, ahem, Endo) into the stress and violence of the Mole Arc, but I think it rattled him pretty profoundly. It's also additionally why her warm greeting hit him as hard as it did: relief across multiple lines, such that he had to remind himself not to relax, despite Yor's apparent return to normal.
And there may be added layers to Twilight's reactions to Yor's bad moods due to his familial history, as pointed out by @unhappy-sometimes in this post; the inverse, of course, is that Yor's general good-naturedness would add layers to Twilight's sense of security with her. And the apparent loss of that, all the more devastating.
Rounding out the original moment though, I think this in many ways demonstrates the point:
Twilight throws away the bug. That is also wild. It isn't like that bug could only be used on Yor; it wasn't somehow modified to only respond to her person. It was a device that could be used and reused on different targets, on people who actually are worthy of being bugged, etc. But instead of pocketing it for later use, Twilight throws it away.
Actually: he not only throws it away, he crushes it first. Perhaps because he couldn't stand to have that particular device around, the device he used when he doubted Yor?
Seems kind of irrational, Twilight.
Seems kind of telling.
I mentioned my last Twilight meta about his relationship with Anya: in that, I suggest Twilight recognised entering into a compact with Anya, which subtly modifies, for him, the motivations around Strix. I think something like that happens here, too. If Yor is willing to go to such apparent extremes to protect him, he'll do his utmost to protect her.
I've had this meta in my drafts for a while, but I'm chuffed by this panel from the most recent chapter, as it kind of underscores all this by Yor's positioning of herself:
(Of course the point is there isn't a dichotomy: they'll protect each other, as indicated by Yor's if I had to choose: she won't have to choose.)
Back to Twilight, at this point, he can still justify all this as being within mission parameters. Of course he should protect Yor: she is an innocent civilian and if anything happens to her it would threaten Strix. But if/when this line is tested, if/when there comes a point where protecting Yor is actually the option that may put Strix at risk or put him somehow in opposition to WISE, then we'll see.
And more importantly, Twilight will see, too.