The best way to approach it is by looking at what Tomura actually does for Yuri. Honestly, he's not just her idol, he's practically her whole support system after that whole thing with her dad. She basically builds her entire initial identity around becoming 'worthy' of him, which sounds unhealthy but ends up being this massive catalyst for her self-discovery. Their friendship is the framework; it's safe, it's pure, it's grounded in a shared passion for music. The love that grows out of it feels so earned because it's layered on top of that solid foundation. It's never presented as one replacing the other, but as the friendship deepening and shifting into something more profound. The fear of ruining what they have is such a huge part of the tension, which makes it so relatable. You see them navigating this change, trying to figure out if adding romance will break the beautiful thing they've already built.
What really gets me is how the manga uses performance scenes to externalize those themes. When they're on stage together, that's when their connection is most visible, and it blurs the line completely between a duet partner's harmony and a lover's intimacy. The art in those moments is just... ugh, it gets me every time. The panels where you see Yuri's perspective of Tomura's hands on the guitar, or the focus on their eyes meeting mid-song—it's all the unspoken stuff. The love story isn't told through big declarations first; it's in the quiet way he adjusts her microphone without being asked, or how she memorizes the specific brand of strings he likes. It builds from a million tiny, friendly gestures into something undeniable.