...good lord some of yall are dense. Did you forget about how Yuu didnt know Yami was still around, let alone Hikaris friend, until literally the argument everybody is freaking out about? Yall need some basic reading comprehension, seriously
This topic has been debated to death in previous chapter posts but I'll tl:dr it here.
Yes, Yuu did not find out that the person he knew as Yami senpai was the same person Hikari was telling him about (Aya-chan) until his confrontation with Yami in the classroom during the festival. Yami hit Yuu with a lot of information during that confrontation. It's understandable that Yuu would be hesitant to mention to Hikari that he used to date her best friend as he wants Hikari to accept his confession.
I think the big sticking point is that people are arguing that Yuu shouldn't have confessed right then, instead he should have told Hikari what was going on. Something like "Hikari, I just found out that the person I used to date is your friend Aya-chan. I didn't know the two of you were friends and I don't want to make things awkward." <--However that scenario is not realistic due to how we've seen Yuu behave in the past.
The best moment to tell her would have been when Yuu left the classroom and found Hikari. We get from Yuu's POV that he realized something was wrong with the way Hikari was acting but he decided to ignore that and just barrel ahead with his confession.
Yeah I just reread ch19 and I can see that. He was pretty evasive, but I can also sort of understand why. It would be a bit of a bombshell for him to suddenly go "yeah I used to date somebody at your school" when iirc she's under the impression that he's never had a girlfriend. I guess he was banking on Aya not being there, since he thought she'd quit school.
As for his relationship with Aya, Yuu is certainly not blameless but in my opinion her actions were far worse and were quite likely to cause him to spiral even more. Most of his issues/shortcomings there were caused by the same insecurity and overthinking that's been going on with him the whole series, along with inexperience and being totally unprepared for a relationship. Realistically he needs to do some serious work on himself before he's ready for a relationship with anybody (a lesson I wish I'd learned earlier). Actually, that applies to all 3 of them.
Overall I'm not trying to say that Yuu is without his flaws, I just take issue with the way that certain people in the forums use those insecurities to paint him as a piece of shit. Like I said in the comment you replied to, the only thing he's done that I think is actually inexcusable is not telling Hikari about Aya at the festival after he ran into her again, and just confessing like nothing happened. Other than that, I can understand what leads to most of his actions, probably because I was in a similarly fucked mental state as a teenager.
Edit: also just as a sidenote, in that other comment I was talking about Yuu's thoughts and actions in the 'bonus' chapters, since the person I was replying to said that those chapters show how much he sucks. I don't disagree that he's a very flawed character overall, I'm just saying that those flaws don't make him an inherently bad person. Aya on the other hand... only a real piece of shit would even consider doing what she tried to pull on Hikari when they first met.
Can't speak for others just myself but I will say my dislike of Yuu comes from the following:
1) He's liked Hikari romantically for a long time but the only effort we're shown is him half-heartedly attempting to get into her high school. Yuu admits it's too late for him to succeed but he needs to try. It left me with the impression that his love for Hikari must not be that strong as he doesn't seem to put much effort in.
2) Once Yuu inevitably fails, he near immediately ends up going to a love hotel with Yami. It's off-putting to hear him talk about how much he loves Hikari but then go for the first girl who shows him any interest. He appears disingenuous.
3) After all of that, when he does end up confessing to Hikari he gives her a whole spiel about how he's always been in love with her. I don't doubt that he has been in love with her and I don't fault him for stating that since there's no way he could know that Hikari witnessed Yami kissing him. However, knowing what we know and knowing what Hikari knows, his confession of always loving her comes off as insincere given how we're only shown two times he's made an effort (trying to get into her school and then this confession) and how quickly he moved on from trying to get Hikari to the literal first girl who came next.
Now, I understand where all y'all are coming from when you discuss Yuu's flaws and his insecurities. I can sympathize with him a bit as I, too, was like that when I was a teenage boy. It's just frustrating to root for the guy when even Yami put more effort into pursuing Yuu than Yuu did pursuing Hikari.
In this manga, we've seen Aya improve from when she first met Hikari. She was going back to school and had made good friends with Hikari, Haru & Yuki. Up until the festival confrontation with Yuu, Aya was showing growth.
We've seen Hikari grow as she came to terms with her feelings for Yuu. We saw her try to pursue Yuu even though a lot of her efforts were for naught. We saw a bit of emotional maturity as she confronted Aya and defended Yuu to her.
We haven't really seen Yuu make much growth as a character. He didn't really learn anything from his pursuit of Hikari. He didn't really learn anything from his relationship with Aya. He did seem to finally start noticing Hikari's actions but even with that he mentions he wasn't 100% sure she liked him and admits he wasn't going to confess unless he was at 100%.
Again, it's just hard to like the guy when you see everyone else putting in more effort. Hell, Haru & Yuki put in more effort by skipping class and tracking Yuu down. They were willing to show up to his school to find him, something Yuu wanted to do to find Yami but chickened out. Even Seki (who in Chapter 12.5 states to her friend that she doesn't even really like Yuu) put in more effort to pursue him than Yuu ever did for the girl he confessed to always love.