Koga used to be quite friendly with Teshima. However, the two had a falling-out over events that took place during the Inter High when they were both first-years. When they race for a position on the Inter High team as third-years, especially during trainnig camp, Koga treats Teshima with disdain, dismissing him as just an "average" cyclist. Despite how bitter their battle was, the two reconciled afterwards. Teshima and Aoyagi embrace Koga and Teshima calls him by his first name for the first time in a while.
While Koga and Aoyagi do not seem to be particularly close, they are shown conversing before the start of the second Inter High. Aoyagi attempts to assuage Koga's worries about Onoda's confidence, saying that Onoda just needs a role to fulfill.
Flashbacks have made it clear that Koga looks up to Kinjou. Kinjou appeared to think that Koga had great potential, which is why he told Koga to use his first Inter High just to gain experience.
Koga is friendly towards Onoda, even offering him advice about how to keep glasses clean during races. Onoda considers Koga a nice and helpful person, which is why he was surprised about Koga's behavior in his race against Teshima.