Yuuichi Kobayashi is the older brother of Megumi Kobayashi who is addicted to online gaming and is also a hikikomori.

He met Sato after his character, Toro Toro, became part of Sa-To's Dragon Hunting Party in the the Online Game "Ultimate Fantasy" and was later on to be revealed as Megumi's older brother after the trio was interrupted by Yuuichi's calling for his sister for food while they were requesting for a "cool off" of the Mouse Road products they have been sold.

(Source: Welcome to the NHK Wiki, edited)