Koichi has medium spiky hair that flicks outwards at the ends where it is parted on the right side and a portion sticks up.
Kouichi is shown to have a very kind and upbeat personality who is always seen smiling and he was very popular in middle school where he befriended Izumi Miyamura in junior high school when many others outcasted him. At first, he was very forgetful and it took him many tries before he got Miyamura's name right (this occurred in one scene - in the manga, he gets it right every time after chapter 18). Kouichi stood up for Izumi when Makio Tanihara told him he was bad news and that he should not hang around him, due to his depressing personality and many piercings. He lets Makio know that Izumi's depressing personality is due to the way Makio's group of friends treat him. Kouichi sees Izumi as a very interesting person and stays by his side, and they become best friends. Kouichi is the person Izumi believes changed him to feel free to be himself around others. Kouichi and Izumi end up attending different high schools: Izumi will go to Katagiri High School and Kouichi will attend Yasaka High school due to his parents wanting him to set his goals higher.
Kouichi and Chika began dating after Shindou confessed to her and she immediately responded with Ok. Chika seems unphased by all the other girls that flock around Kouichi, and has no problem with them giving him chocolate for Valentine's or him giving them graduation gifts. Due to Kouichi being one year behind her, during graduation, she promises to wait 1 year for him. Kouichi has once stated while talking to Izumi Miyamura that he has devoted 60% of his time to Chika. Their relationship is not approved of by her parents, as they think Shindou, who had to repeat his second year, is not an appropriate person for her. The way she treats Shindou now has been heavily influenced by Miyamura, which is now much more forceful. Instead of asking Shindou to stop smoking, she now slaps the cigarette out of his mouth, and, along with Miyamura, they force Shindou to participate to eat cheese in an attempt to get him over his aversion to it.
Izumi was Kouichi's best friend in junior high school whom he has an unusual friendship with. Despite how Miyamura treats him, which is usually with both physical and emotional abuse, Kouichi thinks Miyamura as his best friend. He has a somewhat infatuation towards Miyamura, knowing things like his measurements and etc. It's evident he knows Miyamura well, even better than Kyouko Hori.
Tanihara and Shindou are friends from junior high school. They had a slight falling out after Shindou began spending more time with Izumi Miyamura, but continued their friendship through high school. After Miyamura is also added to their group, Tanihara plays the role of the straight man, trying to keep Shindou (and Miyamura) in check. He sometimes gets tutoring from Shindou, and is described to be a quick learner once it's explained to him.