Mikio Shirato

Mikio Shirato is Yukiko's older brother.

Mikio Shirato challenged Joe wearing a set of Ace AI-powered gear that he designed himself in the first season.

Second season

[Spoiler] He decided to retire after being defeated by Joe. Since then, he has been teaching at a university as a scientist while continuing his research and development activities. [/Spoiler]

(Source: Official Site, edited)