According to some of the comments here, before you get into a relationship with someone, you must warn them upfront that you were previously in a weird relationship with someone who ghosted you then spouted off at you then kissed you without your consent.
If you don't tell them about that before you start dating, you are dishonest. It's definitely not way way weirder to tell people you're not dating about your previous relationships before you go out with them. That definitely isn't crazy person behavior at all.
Someone a few comments before me had it right. This was just not the day to do the confession. Let your head settle. Let the closure sink in first. Also don't leave her on read and lie about why. That's the actual lie. "Lying by omission" because he didn't turn in a book report about his ex before confessing is not the lying here.
But like, yeah. I don't give a detailed report about my exes before I date someone. That would be weird. Even if we're already friends. Going "wait, wait, before we decide to get romantically involved, let me make sure you're okay with everyone I've ever dated first" is a surefire way to make sure nobody ever dates you again.
Dude did not know the two were friends. At all. If he did, then, yeah. "I was in a pretty intense relationship with your best friend" would be important to say upfront.
But also all this shit I'm saying is why this story is so weird.
How are they as close as the story pretends they are when 1. she doesn't know he ever dated anyone 2. he's met literally zero of her other friends
Like if they were actually close enough for all this drama to be meaningful none of the drama would have ever happened.
Like the real story here is that they're not actually all that close. They just hang out because they live next door. And that's the extent of their relationship.
Yuu shouldn't have lied about the phone situation. Yuu shouldn't have confessed that day because that does cheapen it. Mayyybe Yuu could've mentioned the reason he was uncomfortable going to her school festival is that he has an ex who attends.
Hikari is fine. She didn't do anything wrong. She only has the information she has, and to her, Yuu just kissed someone after ignoring her all day then confessed he loved her. Absolutely, turn him down. Sure, you don't have all the information, but even if you did, him confessing that day deserves a rejection.
Yami obviously an asshole. She's gone through a lot but she's still ripping through people's hearts like it's nothing. She's still responsible for her own actions. In this part of the situation. She's still just a kid, too, so to some extent I can forgive it. Kids can do super hurtful insane stuff when pushed as hard as she is. That doesn't mean the other two characters should. You can be as justified for being hurtful as you want, the people you hurt shouldn't have to just let you hurt them.