As far as we know, he doesn’t know. There’s a moment in
chapter 18 where it seems like Yuu might be fishing for information, but that could be a stretch. Also, Hikari mentions that the four of them hung out a lot over break, so Yuu knows about the existence of her three friends, but it’s unclear whether she ever told him their names. I’d imagine she did, but we don’t know for sure.
Thanks appreciate it!
Hypothetical question for those waiting for Yuu's POV:
What would you think of Yuu if it was revealed that—just as Yami realized her ex-boyfriend (Yuu) is actually Ta-kun (the guy Hikari likes)—Yuu also discovered that his ex-girlfriend (Yami-senpai) is none other than Aya-chan, Hikari’s so-called 'best friend'?
to answer this then I do think it'd line up with some of his actions and to me, has them make a little more sense
While I don't think it wasn't the best to not tell her about Yami while there going out I can at least understand why especially after it happened and it likely being a rather touchy subject for him
Like I said he shouldn't of hid it but I I can understand why
What what it REALLY it could potentially change is him lying about where he was during the festival
If he didn't know Yami is her best friend, then I think at that point he had NO reason not to at least open up slightly about what happened even just abit something along the lines of "hey I never told you about this, but a while ago I was seeing a girl and I ran into her and we got into an argument" obviously not EXACTLY but something along those lines
However if he KNEW Hikari and Yami are best friends I think that makes things far far more complicated because now it involves someone else she's close too if it's just a classmate to her, he could explain and not really cause much tension between her and someone else but with her best friend? That's quite different especially when it comes to the topic of the kiss because he basically has two options
1. Tell her about it and say Yami initiated it, and he didn't want it essentially throwing her under the bus and VERY potentially ruining her friendship with Yami
2. Not saying who started it and her potentially getting the impression he was just as culpable
It complicates it...ALOT
All of that said- I still think he should've used this chance to be open that they dated litterally just say "hey I'm sorry i never told you about this but I dated your friend Yami for a while and I ran into her and we got into a fight*
Obviously she still saw the kiss and it probably wouldn't have changed much in the outcomes it would've been far easier to explain eventually why he didn't tell her about Yami