Koji Haruta is a supporting character in the anime Toradora! where he is a male student in Ryuuji Takasu's class, easily recognized by his shoulder-length blue hair. He is regarded by his peers as the class idiot, a reputation earned through his extremely poor academic marks and his relentless, overly energetic behavior. His personality is overwhelmingly cheerful, outgoing, and carefree. He functions as the class clown, constantly engaging in childish pranks, delivering cheesy humor, and indulging in outlandish delusions without a strong grasp of the surrounding social atmosphere. His impulsive nature and lack of filter frequently make him a target of Taiga Aisaka's violent outbursts. Haruta's primary motivations are simple, focused on seeking fun and social interaction. He enjoys making his friends laugh and participating in group activities. He also plays the guitar in a band alongside his friends, revealing a dedicated side beneath his frivolous exterior. Another surprising layer to his character is a hidden talent for writing stories.

In the story, Haruta primarily serves as a source of comic relief, providing a lighthearted counterbalance to the central romantic tensions and character drama. He is a loyal friend to Ryuuji Takasu and Yusaku Kitamura, and he has a notably adversarial relationship with Taiga Aisaka, whom he frequently pranks and consequently suffers reprisals from. His role within the wider friend group helps normalize the social dynamic. While largely a static character whose main function is humor, his hidden talents for writing and music offer glimpses of unexpected depth that subvert his class idiot persona. His notable abilities include playing the guitar and creative writing, as well as a strong resilience that allows him to bounce back quickly from Taiga's physical punishments.