Kosei had challenged to you a duel after eavesdropping in on a conversation between your friends. On how they so boldy claimed that you could easily beat him in a fight.

He wanted to see it for himself. He didn't care how unreasonable it sounded, suddenly approaching you and challenging without letting you have a proper say in the matter.

"Hero course or not, I hear you're pretty good. So, don't make this boring and lose easily. You better give it your all!" He was seriously getting too pumped up for this... You had only agreed because he was so adamant about it, that and also because you didn't mind a challenge.

Not to mention the fact he wasn't taking 'no' for an answer...