Seriously, though, for me it's mostly Shit Vibes coming from him in this chapter and the fact that he completely failed to mention his relationship with Yami to Hikari. Like, we don't know what their relationship is like, and maybe it really is one of those "we hang out with each other because it's easy to fall into this comfortable groove and we can forget the rest of our real lives and relax and not talk about what's stressing us out" kind of deals.
But my guilty-until-proven-innocent of hiding a relationship from a female friend is that he doesn't want it to hurt his chances with her, and by the time he's no longer into Hikari (IF he's ever no longer into her,) the ship has kinda sailed and it's awkward to go "Hey, funny story about my doomed whirlwind romance with your senpai..." It's just that it's the kind of thing where there's no way Yami wasn't the most important thing happening in his life at that time, and when you're that in love, it takes a superhuman effort to not make it the only thing you talk about. Even Yami was sick of hearing about Hikari from him, so the fact that it didn't go the other way is kinda eh. Either way, though, it's fundamentally about misrepresenting himself to Hikari, and I expect better of him. As far as we know, Hikari wouldn't do him like that.
Edit: Like, seriously, huge credit to Yom for subtly making it such that Yuu's relatively innocent-seeming words and behaviors during just a few panels give me such fucked vibes of "oh I see you, you piece of crap."