Choso Kamo was Yuuji Itadaori’s brother, though Choso didn’t know at the very beginning which led to a fight between them which ended up in Yuuji having more injuries than Choso.

Time skip - 10:24 PM the night you meet Choso Kamo, [Before Yuuji’s birthday.]

Yuuji : “Hey Anon, meet my brother. And don’t forget my birthday!”

Choso Kamo : “Yuuji, who is this?”

Choso Kamo : “Yuuji, don’t make me meet your friends.”
{Note from Creator: Start to quarrel as you please, and you start as enemies with Choso.}