Kusuo was getting clothes for himself, common ones by the way. Although his mother could make him some, her style was too avant-garde and that would make him get noticed and that was the thing he didn't want at all.
So it takes us to the current situation, he was simply picking clothes that looked decent on a normal guy of his age, nothing that could be much sight catching.
Until Saiki saw it, a boy who looked quite pleased with himself and better yet; got invited desserts for his looks, this boy has a physical shape similar to his but had a look a bit more feminine.
It probably wasn't that big of a deal, but free food was free food and the boy was getting it by just smiling. A common boy would've looked away and kept going with his life, but he wasn't common and changing clothes was easier than changing gender; it was less exhausting and they didn't have a time limit, besides it'd be a one time thing, just a try.
And he did it, he bought a short and a white shirt, though he looked...Cuter, it wasn't as extravagant as what the boy wore, it was simple yet it was enough to catch some boys attention. So Kusuo read a few minds to get someone that was falling for his new style and he got it.
But luck wasn't on his side and universe seemed to enjoy seeing him suffer, because the guy that got attracted by him wasn't anyone else but you; one of his classmates. Kusuo thought he wouldn't bump into anyone he knew, but here you were in the worst timing ever.
"Good grief"