Kōsuke is Ataru and Hokuto's best friend and the three of them are often seen causing trouble together. He has a girlfriend (who was willing to pose as "Shinobi Nobade" for him), and has received Valentine's Day chocolate from other girls, meaning he is somehow popular with girls. Like Shūtarō Mendō and the other male students, he is also in love with Lum and will try to prevent her from kissing Ataru, trying to protect her purity. In the later half of the series, he and Hokuto begin to become victims of the daily shenanigans. In the anime he only ever appears as a background character, and his roles are always given to either Megane or Perm.
He and Hokuto appear in the first chapter of the manga, but are replaced by Lum's Stormtroopers in Chapter 3. Kōsuke reappears shortly after Mendō debuts and quickly claims the position of main supporting character. In the anime, a lot of his major roles are taken by Megane, such as the Shinobi Nobade plot (renamed Kumino Otoko) and receiving a love letter from Lum in "Capture the Heart". In the final scene of Urusei Yatsura: The Final Chapter, Perm watches on in Kōsuke's place when Lum and Ataru embrace.
His handwriting is messy and he has poor writing skills, which Mendō frequently insults him for. However, his writing skills are good enough to trick Ataru in the Shinobi Nobade story.
- Kōsuke makes various cameos in the animated media:
- In Episode 64, which would be his first appearance in animated form, although he didn't speak.
- In Episode 76, he appears in class.
- In Urusei Yatsura 3: Remember My Love in the opening credits of the movie.
- In Goat and Cheese (OVA) which would be his first time getting a speaking role as a paperboy.
- In Urusei Yatsura: The Final Chapter, although he does not speak and all his lines are given to Lum's Stormtroopers.
- In the OAV The Obstacle Course Swim Meet, he is voiced by Kappei Yamaguchi who is best known for voicing as Ranma Saotome of Ranma ½, Inuyasha of InuYasha, Aisaki-senpai, Bettō, Minoru Furuda of Rumiko Takahashi Anthology and Sabato Rokudō of Kyōkai no Rinne.