I mean… is it really that creepy to go looking for your girlfriend who just ghosted you without a word? The message is pretty clear, sure, but if it were me—and I was 100% committed to the relationship—I’d at least want to know why. Because of that, and what Yuu said when he confessed to Hikari, it feels to me like he was never fully invested in Yami.
Other than in Yami's delulu mind, Yuu had no way of finding her or getting in contact.
They had no mutual friends, he didn't know where she lived, just her phone number (and presumably her line) so if she cut those off he couldn't go find her unless he went creeping around her school - and even if he had, she wasn't going. (Her skipping school [so they could go fuck] was something he knew about her as well)
Yami was hanging at the station to her school, after avoiding it for a long time (I think she said a couple months), dreaming that Yuu would come find her again. Yuu only stopped at that station the one time because he took the entrance exam (and failed), and so even if he went to the station to try to find her, she would have been MIA.
She wanted him to be so utterly hung up on her he'd hang around where they met the one time for months.
She didn't say anything when she cut him off, gave him no warning. If he would have gone creeping around his school or moping at the station for the couple of months needed to encounter Yami again, he'd be getting shit on for not being able to taking a hint by the same people who claim he didn't do enough to try to find her.
Yami is just completely self-destructive, and its a shame for Yuu and Hikari they got caught in her blast radius.