Kōji Watanabe is a seventeen-year-old high school student and one of the two main protagonists of Shakotan Boogie. He is in the same grade as his close friend Hajime Yamamoto, though he consistently refers to Hajime as "senpai" and looks up to him. Kōji does not own a car and instead rides a Yamaha Champ moped, usually riding as a passenger in Hajime's lowered Toyota Soarer during their nightly outings. He has a gentle, kind-hearted personality and is noticeably more reserved and level-headed than the impulsive Hajime. Despite being good-looking enough to attract attention from girls, Kōji remains largely unaware of his own appeal and rarely initiates romantic encounters. His primary motivation is to accompany Hajime in their shared pursuits of cars, street racing, and socializing, though he often takes a supporting role. In the story, Kōji functions as Hajime's loyal sidekick and moral anchor, providing a contrast to his friend's brashness. His key relationships include his deep friendship with Hajime, whom he admires, and his romantic involvement with Yumi Saitou, who develops strong feelings for him and actively pursues him. Over the course of the series, Kōji gradually becomes more conscious of his attractiveness, though he remains humble and faithful to his friendship. He has no standout abilities beyond his easygoing nature and pleasant demeanor, relying on his friend's car for mobility and adventure.