Eri can't be blamed for not knowing how Fuyuki feels. But what about not letting Fuyuki date or letting other people touch her? What about kissing Fuyuki while she's in a relationship, knowing that she likes girls, and then claiming that you don't have romantic feelings for her?
First of all, let’s address the kiss. Eri was emotionally vulnerable and hysterical at the time, and acted on impulse. It was bad, she knew almost immediately that it was a mistake, and she apologised at the first available opportunity. Of course, we also have to acknowledge that her perception of why it was bad is different from the reasons why it hurt Fuyuki. That’s why her sincere apology ended up making things worse, because she didn’t know and couldn’t have known the whole situation.
As for Eri’s so called “possessiveness”, I think this is people misunderstanding what it means when Eri says that Fuyuki is a her number 1 priority and vice versa. Eri has dated plenty of guys before, but she has always prioritised Fuyuki. That’s not just wanting Fuyuki, her most important person, around on special occasions like her birthday, or asking her for help when she needs it, she also wants to do the same for Fuyuki. If Fuyuki asked, Eri would have dropped any of her boyfriends at the drop of a hat to go to Fuyuki’s side. She’d probably give up dating altogether if Fuyuki ever expressed having a problem with it. And when Fuyuki said she got a “girlfriend”, she claimed at the time nothing would change about their relationship.
Unfortunately, Fuyuki was reaching her limit and used Kyou and Kisaki as excuses to distance herself from Eri, which made the latter naturally anxious at the seemingly sudden change of behaviour. From her perspective, it looked like Kisaki was monopolising Fuyuki‘s time to steal her away from Eri, something she’d never purposefully let any of her boyfriends do. She got mad at Kyou when he claimed to be the reason why Fuyuki chose not to be there for Eri’s birthday. And in Eri’s defence, Kisaki also played up the part of the possessive girlfriend for the sole purpose of provoking a reaction and follow-up introspection from Eri.
As for the drunk Fuyuki incident, like I said, part of prioritising Fuyuki means being the one who is always there for her when she’s the one who needs help. That‘s how they were when they were younger, though it’s happened less since they grew up as Fuyuki has gone out of her way not to show her vulnerable side to Eri. Now consider from Eri’s perspective, Fuyuki is in an incrediblely vulnerable state that she isn’t even aware of her surroundings and can’t speak for herself (which is why Kisaki could get away with what she did), she’s Eri’s most important person and vice-versa, it makes perfect sense to her she’d be the one to look after Fuyuki. Then Kisaki played the girlfriend card, in public no less. Leaving her with anyone in a
lesser relationship feels like forfeiting something precious, and also like a betrayal of Fuyuki’s feelings, and yet, she knows her perspective isn’t supported by the majority.
Even a lot of commenters can’t fully relate to Eri’s perspective and pass it off as simply just possessiveness and from an outside perspective that is how it appears, however really it’s because of how much she cherishes Fuyuki that she doesn’t feel she can entrust her to anyone else if it can be helped. If she were a family member or significant other, most people would side with her. Heck, some people even expect SOs to act “possessive“, otherwise it‘s like they don‘t care about their partner as much as they should. And again, as has been stated repeatedly, because all of Eri’s experiences with romance have been lukewarm at best, she doesn’t have a high opinion of
those relationships when comparing it to the one she’s shared with Fuyuki, a perspective that isn’t uncommon for someone on the ace/aro spectrum (I suspect she‘s demiromatic, possibly demisexual). It’s Eri’s rather uncommon mindset that makes it difficult for a lot of people to understand her actions and intent, viewing her actions only from the perspective of an outside observer.
If I’m honest with myself, it is quite fascinating to observe from the comments. I wonder how much of it is a lack of media literacy, and how much of it is an inability or unwillingness to understand a perspective so far removed from one’s own. I’m very socially awkward, and struggle to connect with other people, so being able to get inside a character’s head, to understand their motives and actions is perhaps something I’m more inclined to do.