I kind of faintly remember Yoshino saying that he still has a leather jacket his big brother used to own. But I can’t recall when or where he said it. I’m not even 100% sure it was Yoshino.
As far as influence goes, I think Yoshino simply has a chuunibyou-like obsession with the cool, wild delinquent style.
Yoshino probably made a surprising appearance in a certain route, namely Terra.
There was a scene where Kotarou saw some punks bothering a kid for money, so he went in and beat them up. That kid proceeded to ask him about… leather jackets and outlaw things and Kotarou, feeling funny, answered all questions in a fashion that made him look really uncool, so the kid ran away with a broken heart. You guessed it, I assume that this child was probably Yoshino. Incidentally, Kotori witnessed the whole thing, so she gave Kotarou a dirty look for using violence.