I'm honestly a little surprised how often this comes up—maybe it's because the manhwa adaptation feels so different from the original webnovel? The main duo is Haebom and Taesung. They've lived together since Haebom's parents died when he was a kid, so Taesung's family took him in. They start off as incredibly awkward, almost estranged 'brothers' under the same roof, with Taesung being cold and Haebom being shy and sort of tiptoeing around him.
A lot of the early story is that classic slice-of-life tension, them navigating high school and their forced domesticity. The 'cherry blossoms' part really kicks in as the seasons change and their relationship slowly thaws from ice-cold to something tender. The side characters like their friends Joowan and Sua matter, but the heart of it is just watching those two figure out how to be in the same room without all that painful history hanging between them. It's the small moments, like sharing an umbrella, that get me.