It's funny, I always find myself drawn to writing the quiet moments with him. Everyone jumps to the volleyball court or the intense matches, which are great, but I think his character really opens up in those mundane, in-between spaces. Like, imagine writing a scene where the reader is studying late in the library and Kageyama just... sits nearby, methodically organizing his own notes without a word. The engagement comes from the unspoken tension—the way he might glance over at a difficult problem, hesitate, and then gruffly point out a mistake. It's all in the subtext and the tiny physical details: the way his fingers tap against the table, the slight frown of concentration that looks like anger but isn't. That restraint makes any small breakthrough, like him actually explaining something clearly, feel huge.
Another angle that works is leveraging his competitive nature in non-sports contexts. A scene where the reader challenges him to something trivial, like who can finish their milk bread first or a timed puzzle game, can be surprisingly electric. You get to showcase that single-minded focus turned onto something silly, and his utter seriousness about it is both hilarious and weirdly charming. The key is to not soften him too much; let him be blunt and socially awkward, but show his actions speaking louder. Maybe he finishes first and then watches the reader struggle, not to mock, but with this intense, analytical stare, before wordlessly sliding over a hint. The 'reader' character needs to be proactive enough to poke the bear, so to speak, or the scene just sits there.