— Koyama's signature laugh
He wears black framed glasses and appears to have crooked teeth. His hair is black, long, and shaggy, giving him a disgruntled look. His face is often contorted with an overly large smile and squinted eyes, accentuating his strangeness.He also looks to have a unibrow. Also with uneven teeth.
Koyama is not different from the rest of the Five Virtuosos in terms of behavior and temperament. He, like most other people in the main campus, looks down on the E class and is constantly frustrated when beaten by them. He is an extremely intelligent person, and excels in science, despite being constantly beat out for the top spots.
Memorization: He claims to have memorized every chemical formula in preparation for Kunugigaoka's exams. His abilities are lampshaded further in Koro-Q, where he claims to have memorized every monster listed in the monster encyclopedia.
(About Gakushu Asano) "Being honest; he could do many things he finds gross."
- The name Natsuhiko means "summer" (夏) (natsu) and "boy, prince" (彦) (hiko).
- Natsuhiko's surname Koyama means "small" (小) (ko) and "mountain, hill" (山) (yama).
- The character book  describes his middle school life as unpleasant until he meets Asano, who tells him “With your strength, you should more boldly look down on others". Thanks to that, Koyama became much more confident, unfortunately to the point of being egotistical, but he feels a strong obligation to Asano because of this.
- According to the Graduation Album , Koyama later finds employment at an unnamed pharmaceutical company.
- While working there, he chopped off his long hair because it got in the way while developing new drugs, which awakens him to the call of beauty. He ends up not only straightening his teeth, but even goes as far as to get a mandible reduction. He might be a hottie now.
|Kunugigaoka Junior High School|
|Faculty||Korosensei • Tadaomi Karasuma • Irina Jelavić • Aguri Yukimura|
|Students||Masayoshi Kimura • Hinano Kurahashi • Yuma Isogai • Hinata Okano • Hiroto Maehara • Meg Kataoka • Kotaro Takebayashi • Toka Yada • Koki Mimura • Rio Nakamura • Nagisa Shiota • Kaede Kayano • Taisei Yoshida • Yukiko Kanzaki • Taiga Okajima • Rinka Hayami • Tomohito Sugino • Yuzuki Fuwa • Takuya Muramatsu • Kirara Hazama • Ryunosuke Chiba • Manami Okuda • Sosuke Sugaya • Sumire Hara • Ryōma Terasaka • Karma Akabane • Itona Horibe • Ritsu|
|Faculty||Gakuho Asano • Kiyoshi Terai|
|Students||Tomoya Seo • Teppei Araki • Natsuhiko Koyama • Ren Sakakibara • Gakushu Asano • Kaho Tsuchiya • Kazutaka Shindō • Kokona Tagawa • Nobuta Tanaka • Chousuke Takada|