Koseki is a man who used to be a member of the Sports Cycle Hamada Cycle Team along with a 13-year-old Kinjou Shingo.
Koseki often scolded Kinjou, but did so because he believed Kinjou had potential. Despite his young age, Kinjou was the only one who made the effort to keep up with Koseki during practice. Kinjou looked up to Koseki as an idol of sorts and even wanted to ride the same type of Trek bicycle as him. He also dreamed of competing against Koseki in a race and being victorious.
While Koseki has a laid-back air off the bike, he was often strict with Kinjou and put a great emphasis on teaching him respect.
In the bonus strips of Kinjou's Spare Bike, he's revealed to have the ability to assess a bicycle from a single glance, before laughingly pointing out the ability is fundamentally useless during a race.
He's concerned that his bangs may be growing too long and has a habit of blowing them out of his eyes.