Building Sohoku's Cycling Team
Koga is introduced in Ride 15, though he remains as a side character until the second training camp of the series. He advises Onoda Sakamichi against trying to ride rollers for the first time, and is surprised when Onoda manages to ride the rollers fairly easily.
In the manga, Koga is paired with Sugimoto Terufumi and introduces himself in a friendly manner. Sugimoto mistakenly thinks the practice is about building the new first-years' confidence but quickly realizes that it's in fact to show the new members their weakness. Sugimoto returns to the club room to exclaim how amazing Koga is.
Sugimoto is paired up with Teshima Junta in the anime instead.
Second Year Events
Koga becomes a major character during the second training camp. He threatens to replace Teshima Junta on the Inter High team. It is revealed that he participated in the Inter High as a first-year even when he only rode 980km of the 1000m in training camp, but ended up crashing badly. With both he and Kinjou retiring, team Sohoku lost its chance at victory. Also, due to the injury he sustained, Koga lost his chance at returning to training camp, let alone competing as a second-year. It actually cost him 1.5 years (18 months).
Koga is comparable to Imaizumi Shunsuke and Naruko Shoukichi in that he is seen as a "elite," and he looks down on Teshima, who is just an ordinary person. Despite this apparent difference in skill, Teshima manages to beat Koga and protect his spot on the Inter High team. Afterwards, Koga reconciles with Teshima and Aoyagi, though he is of course upset that he lost his final chance at redeeming himself in the Inter High. Despite being upset over losing, Koga notices that Kaburagi Issa is suited to being a sprinter rather than an all-rounder. He tells the others about his discovery, but doesn't actually mention it to Kaburagi.