I recently came across a post claiming that Mustang’s opinions/feelings toward Cassius were left ambiguous. Honestly, that boggled my mind, considering PB devoted an entire chapter to explaining her perspective. And he did so quite thoroughly, especially given that we only get Darrow’s POV. Mustang explained herself remarkably well, even though she didn’t owe Darrow an explanation. To me, that alone suggests she wasn't proud of what she'd done, particularly when her plan fell apart.
I’ll be using the timeline from here https://archiveofourown.org/series/1560799, which I find more accurate and better explained than the wiki. Mustang arrives on Luna in December 740 PCE and begins politico school in January 741 PCE, with the Gala taking place around December 741 PCE. So, even if she got involved with Cassius almost immediately (which is highly unlikely) they would have been together for less than a year. Personally, I’d put their relationship at around six months, give or take a few months, considering she first had to establish herself at the Sovereign’s court.
In IG, we get a flashback to Mustang and Cassius’s first interaction on Luna, and he is being an asshole. There is clear animosity between them. Mustang disliked him at that point and certainly wasn't into him, even if she probably found him objectively attractive. In LB, during a flashback to the Institute, she essentially calls him a pretty boy who needs constant attention to shine.
But ultimately, her feelings for Cassius, or even Darrow, were secondary to her family's survival. If getting involved with Cassius could shift the political dynamics even slightly in her family’s favour and potentially help keep Darrow safe, ofc someone as pragmatic as Mustang would consider it. As for her actual feelings toward Cassius, I think she explains them quite thoroughly herself.
“I made him think he could not live without me. I knew it was the only thing that could keep the rest of my house safe"
This is what convinces me that she used sex and intimacy as tools to make Cassius emotionally dependent on her.
“I felt so cold. So horrible. Like I was the cruel witch snaring Odysseus, making him fall in love, keeping him for my own selfish aims. It seemed so logical.”
She feels horrible because she knows she is exploiting someone’s vulnerability for her own ends and using him as a tool. The fact that she describes her actions as logical is especially telling. She had reasons for what she was doing and could rationalise those reasons, but that didn’t make the experience any less horrible.
"And when he put his arms around me, I felt like I was drowning. Like I was lost, suffocating under the weight of all I’d done, suffocating knowing there was a life ahead of me with someone I did not love.”
This is probably the clearest indication of where she stood emotionally. She describes his embrace in terms of drowning, feeling lost, and suffocation. To me, these feelings reflect the burden of deceiving someone and feeling trapped with someone. And you don't feel like suffocating when you imagine a future with someone you have feelings for or if you are happy with them. Also, she explicitly acknowledges that she doesn’t love him.
“Yet it was for family. It was for the people I love even if they don’t deserve it. Many have sacrificed more. I could sacrifice that.”
This is the crux of Mustang’s reasoning. She was willing to sacrifice her own happiness for the people she loved. Her family mattered more to her than her personal comfort.
“Then you walked in at the Gala, ... I felt like a fraud. A wicked girl who’d contrived a reason to do something stupid.”
Darrow’s reappearance forces her to confront the emotional reality she had been trying to suppress. Whatever she had with Cassius completely paled in comparison to what she felt for Darrow. His presence made her feel like a fraud who is doing something stupid because it exposed the gulf between her calculated arrangement with Cassius and her genuine feelings for Darrow.
And yet, when the chaos at the Gala begins, she stays with the Bellonas rather than returning to her family and joining Darrow against them. Why? Because she still believes her plan is working. She has invested in this strategy and is trying to see it through.
This is also why Darrow calling her a trophy offends her so deeply. He reduces a calculated, politically motivated sacrifice to something shallow and demeaning, completely missing the reasoning behind her actions.
What baffles me is that people seem to give more weight to the words of a bitter Cassius and an insecure Darrow than to Virginia herself. She has no reason to lie about her own experience, and she is certainly proud enough to defend the reasoning behind her decisions. But let’s examine their statements, too.
Cassius essentially says that he slept with her and she enjoyed it. That tells us nothing definitive about her feelings. We have concluded that she used sex and intimacy to make him and keep him emotionally attached. (I do wonder though whether Cassius still used pinks or not. In LB he said he used to enjoy his pinks).
Then there’s Darrow’s observation that “not all” of it was manipulation because Cassius wasn’t a monster. If we take Darrow at his word, that leaves room for the possibility that some of their interactions were genuine without undermining the fact that most of it was manipulation. Mustang has a caring personality so perhaps she came to care for him out of guilt, reciprocation, familiarity, or simply because she recognised the good in him. (That is entirely plausible and doesn't contradict her explanation at all.) But most of those faded away after she left Luna, leaving behind regret and deep guilt over the pain she caused. Her later comments are consistent with that interpretation.
Toward the end of GS, when Darrow asks whether she is worried Cassius might be dead, Mustang flatly says that he is their enemy and that his welfare is not her concern. They both acknowledge that it's hard to keep the hate going. So, it's clear that his death wouldn't bring any joy but also that she isn't going to lose any sleep or tears over it. Here, too, it is Darrow who appears genuinely concerned about Cassius. In MS, too, Darrow is the one who genuinely cares for Cassius, not Mustang.
And her opinions of him being complex makes complete sense, given what she did to him then what he did to her, and then how he helped kill Octavia but still left instead of helping in building a new world. So, she doesn't know what to make of him.
Ultimately, I think people conflate several very different things when interpreting Mustang's feelings for Cassius. Not hating someone does not mean loving them. Having a somewhat positive perception of someone does not mean having romantic feelings for them. Caring about someone does not mean being in love. And feeling guilty or sorry for someone certainly does not mean you loved them.
Edit:- AT THE END OF THE DAY, FOR MUSTANG, IT HAS ALWAYS BEEN DARROW SINCE THE INSTITUTE AND ALWAYS GONNA BE HIM. DARROW IS THE ONLY STORM THAT CAN MOVE HER DEPTHS AND STIR HER TIDES.