Sai would be pretty delusional about his feelings for you. He still lacks the capacity to understand what he’s feeling, especially when it’s towards you. Emotions are very complicated and confusing for him, but it’s even more puzzling when he thinks about how he feels towards you. He couldn’t understand why he felt differently about you than the others and Sai couldn’t quite tell what he was feeling.
So obviously he’d probably ‘research’ about these feelings in the library. After reading some books that have terrible advice about romantic feelings and dating (not that he’d realize that lmao) he’d finally come to the conclusion that he has a romantic interest in you.
(Also random, but I feel like Sai would probably offhandedly mention this research he did to Sakura or Naruto and they’d be like ‘??? Obviously you have a crush? What did you think it was?’)
Anyways, I feel like he’d be a really big stalker. And he’d be extremely good at it too considering he is a former Anbu member. He finds it interesting to see how you act when you think you’re alone versus how you act around him or others. It’s very interesting to Sai to see the different aspects of your personality this way.
Sai doesn’t understand feelings, but he’s still intelligent. He’d realize that this behavior wasn’t a totally normal thing, but he wouldn’t really think that it’s a bad thing because he doesn’t view it as harmful. Anbu stalk their targets all the time and that’s normal, in his mind this behavior is pretty typical stuff.
I think that Sai would take things you do the wrong way pretty often. He’d probably read something in a book about how to act romantic or something and take everything it said to heart. Like oh you’re wanting to go to lunch with him? Well this is obviously a date because the book said people who are trying to pursue someone ask their crush out to lunch or something. Or oh perhaps you might be unconsciously maintaining eye contact with him more than usual today? You’re obviously in love with him because his book said this would happen.
Sai would also try to make romantic gestures to you, but it’d probably fly right over your head because it didn’t make any sense. Like Sai would do what he thought was the most romantic thing ever and you’d just be like ‘oh… okay. thanks’ and he’d get so confused. He’d probably then just bluntly state his ‘feelings’ for you.
When you do presumably enter a relationship with him, it is 100% up to you to lead the relationship and know what to do. Sai wouldn’t have any idea what to do, it’d honestly be so awkward at the beginning.
He’d probably expect you to tell him anything and everything about yourself. He’d also probably follow you around all the time now because he’s your boyfriend. Isn’t that what he’s supposed to do? He’d become kind of obsessive towards you. He was like that before he started to date you, but it’s now he’s much more open about it. He’s your boyfriend so obviously you’re supposed to tell him anything he wants to know about you? That’s how it works, why don’t you realize that?
Being mean is something Sai often finds himself being, intentional or not. Sai sometimes says things bluntly in a way that might offend you. He doesn’t always mean to be rude, but he doesn’t find a reason to censor himself. Though there are times he intentionally says rude things to you as well. Sometimes you just need a reality check, he thinks. You should have the sense to listen to him or to not leave him or else he will tell you so.
Manipulative is definitely something that Sai can be at times. He doesn’t completely understand how he feels towards you, but he knows he wants to be with you. And if he feels he has to ‘push’ you slightly to be with him then so be it.
I think he’d also be quite jealous at times, even if he doesn’t realize it himself. Sai would just feel such emotion when you spent too much time paying attention to someone else when he was right there. He didn’t quite grasp the idea of being jealous, but definitely doesn’t like feeling that emotion. So stop it, will you? You acting like that is messing with his head and he doesn’t like that at all.