Medb. Holy shit it's so glaringly obvious.
Don't leave yet, sit down, strap in, we need to start getting into background and interactions and t h e m a t i c s.
Okay, so, for a summary in Medb's history for those who haven't gone beyond the point of no return: broad strokes starts us off with Medb married to Conchobar, but their marriage fucking sucked even though they had a kid and she left him. Medb's dad decided to just marry one of her sisters to Conchobar to replace her, but Medb murdered her because the sheer fucking gall of thinking she could be replaced, so her dad got it into his head "hey, marrying her off might be uh, detrimental" so he just invaded and deposed the King of Connacht, installed Medb in his place and dumped her there.
So with the king as her lover but no say in things, she rules undisputed. Flash forward, Conchobar and her meet again, Conchobar assaults Medb, her husband challenges him but loses, forcing Medb to flee. She takes the commander that got her to safety as husband and has an affair with Ailil mac Máta (who you might know as the guy that was responsable for Fergus' death). When the new husband finds out, Ailil kills him and becomes the next husband.
From that point onward, Medb has a fuckton of lovers and basically does whatever the fuck she wants (including invading Ulster for a bull and engineering the death of Cú Chulainn) until the point of her death. You might know these storied events as the Táin Bó Cúailnge.
Okay, very, very broad strokes background gotten over with. Observe that Medb's life from the start was defined as being a bargaining chip and she very clearly did not like it, then she was dropped like a hot potato when she made that displeasure known. If that weren't enough, when you look past the sex, you also see a very defining aspect of her character: Medb is absolutely addicted to being in control of everything around her.
She loves using people, she loves having power over people, because that is really the only way she knows how to interact with people. Her entire background is full of backstabbing, manipulation and falsity, so obviously she defaults to that in every single interaction. Justice and miracles don't exist, the only thing you can trust is yourself and your capacity to outplay everyone around you.
That is why she clashes and becomes obsessed with Cú so much, because Cú has done the single most unforgivable thing in Medb's eyes: he didn't hate her.
She wanted him to love her, to submit like everyone else, because that'd mean she had won, but he didn't. That's okay, that's alright, being loved is ideal, but she can work with that, after all if someone hates you that means you can work around that hate and still come out on top. Yet despite everything she did, from invading his country for the pettiest reasons to causing him to fight his best friend to the death, Cú still doesn't hate her because as a person, he's really weird by our standards and that sort of thing just isn't a reason to hate somebody. He doesn't like her, but he also doesn't hate her. He's just "meh, annoying."
Love is ideal, she could deal with hatred, but indifference is absolutely unforgivable, and this all stems back to that addiction to control.
Her own idea of what "love" is has already been tainted by that, because she defines being loved as being put on a pedestal and everyone offering up their everything to her alone, and that is why she is so obsessed -- and in later My Room lines you actually see her doubting that, because every interaction she has ever had has been colored by the fact that people wanted to use her -- for her power, for her ruthlessness, for her kingdom, for her smoking hot body -- so she doesn't know what a "genuine" thing is at all. Even in her interlude she cannot fathom the idea that someone'd want her because of who she is, she shoots down the idea that she is a good person deep down, and even if she wants a husband that loves her unconditionally and can be described as the ideal, caring man who is not mean, who is not afraid and who is never jealous of her, she will never work to become the kind of person that deserves someone like that even in the slightest.
And that is why Cú will never fall for her, because even if she has an iron will, she will just never work to improve herself, because her issues are for everyone but herself to deal with, as opposed to Cú going the "my issues are my own." on the other end of the spectrum. They are similar, but also absolutely incompatible in the ways they do divert.
So we take someone like this -- an objectively terrible person who doesn't want to be better, who is awful but also strives for the genuine nonetheless -- and you drop her with someone like Shirou who does everything for the sake of others and whose entire reason for improving himself is being more effective at doing so, whose desires are still at this point something he took up from someone else instead of developing his own.
How does she even deal with this?! Who the hell is this guy to come in and do these things and be this way? Regardless of the route Shirou goes through, there is an intrinsic selflessness to himself that both Medb and Cú lack, so there isn't even the unexplored common ground of defining yourself through your accomplishments and victories and pride, because he just doesn't. He is in some ways even more distant to her than Cú ever was, and that'd drive her nuts because it isn't like this is some random guy she can just ignore -- this is her anchor, this is the one that bears the Command Mantra, this is the man that called her to the modern era.
And he defines worth as how much he can do for others, honestly and almost self-destructively, where Medb defines worth as how much others can do for herself to the point they break.
So how does it develop? Is the endgame two diametrically opposed viewpoints clashing and leaving only seething hatred, is it Shirou understanding that being selfish doesn't need to be so bad, is it Medb understanding that she will never have what she wants unless she changes up her act? Is it them having this passive-aggressive dumpster fire where they each try to influence the other?
This is so awful.
I love it.