Kōsuke Sakaki, once Kakeru Kurahara’s teammate on Sendai Josei High School’s track team, holds a profound grudge against him. This stems from a violent altercation where Kakeru assaulted their coach, resulting in the entire team's one-year competition ban – an outcome Sakaki solely blames on Kakeru.

Now a first-year student at Tokyo Sport University and member of its track team, Sakaki sports short red hair with bangs parted down the middle. He demonstrates loyalty and friendliness toward his current teammates. Yet his demeanor shifts drastically around Kakeru, whom he actively provokes and seeks to spite, deliberately targeting Kakeru’s emotional state during competitions.

Sakaki reemerges as a rival during university track meets, facing Kakeru, now running for Kansei University. In these races, Sakaki mercilessly taunts Kakeru, invoking their shared past and the damaging consequences of Kakeru’s actions. This calculated antagonism positions Sakaki as a recurring challenge, forcing Kakeru to confront unresolved resentment from high school.

His name reflects symbolic meaning: "Kōsuke" combines the kanji for "prosperous" (浩) and "forerunner" (介), while "Sakaki" (榊) signifies a sacred Shinto tree.