This is a bit ooc but I have two ideas as to how this could go. Noble/Royalty au where either Sae and Rin are arranged to be married to 'Keep the bloodline pure' or Sae just marries her out of his need to control her and keep his power. Either way, Sae could care less about her and their marriage. Most days he can't be bothered with her, he spends a lot of time away from the mannor just to get away from her and attend to things he finds way more important. He routinely cheats on her on those so called 'business trips' too. Rin on the other hand is absolutely whipped. The first thing she does when she wakes up is shower him in adoration whether it be kisses or hugs because a simple 'good morning' doesn't do it for her. She insists that she feed him when they eat together, that they bathe together and travel together, all the things expected of a regular couple.
Rin day dreams about him whenever he's out. She makes him little trinkets and writes about him in her diary entries and poems as she anticipates his return, just waiting to shower him in adoration all over again the minute she catches a glimpse of him. She's so blinded for her love for him that she either thinks of Sae's obvious lack of interest in her as him being his usual gloomy self or just doesn't see it at all. She somehow manages to look past all the flinching when she touches him, the excessive excuses as to why he can't spend time with her, the disheveled appearance he has sometimes after returning home, her having to lead their conversations, the way his mood and interest seems to shift when he's around other men and women. It gets to the point where by the time she finally realizes it, when she finally stops letting the delusions cloud her judgement, she gets desperate for his attention. She tries so hard to appeal to him, wearing her makeup a certain way, dressing differently, trying to engage in conversations about stuff she thinks he would care about. She somehow comes to the conclusion that Sae hates her because she's not offering herself to him. Rin confronts him one night, half naked and needy, begging him to fuck her. She has a firm grip on his arm, grinding her body against his and swearing that she'll finally be a good wife, that she'll do anything that he wants her to and that she'll give him a healthy set of heirs. She'll do anything if it means he'll have some interest in her, if it means he won't leave her. Sae is just as uninterested as before, now slightly disgusted, but he still goes on to pity fuck her within an inch of her life. If he can give her a baby maybe she'll love and dedicate herself to that. Maybe she won't be as lonely.