Takebayashi has black hair combed over along the sides of his head and wears round eyeglasses. His uniform consists of a suit jacket worn over a dress shirt and a black tie, as well as dress pants and dress shoes. He looks similar to the persona of manga artist Mizuki Shigeru, enough for Korosensei to be spooked by the resemblance.
Takebayashi is mostly a quiet person. However, he seems somewhat eccentric in his behavior and stands out as the oddball in the class. He also seems to be clumsy at physical activity, having ping pong balls hit his glasses while playing with others and nearly losing balance in one of Tadaomi Karasuma's lessons. He initially wanted to leave Class E and seemed ashamed to be in the class but decided to stay when he realized how much his class and Korosensei cared for him and vice versa. He is also obsessed with anime, at one point stating "A girl isn't a true girl until she loses the third D" implying he likes anime girls more than real women. He also enjoys going to maid cafes.
Takebayashi's family is extremely wealthy and elite which has been producing doctors for generations. His elder brothers are all in the University of Tokyo in the medical departments and because of that, there is enormous pressure for him to succeed and since falling to Class 3-E, he is currently considered to be the outcast of the family.
After the assassination of Koro-sensei, Takebayashi was last seen working with Okuda to expand the medical research based on Korosensei's legacy using revitalizing slime recipes that can make an universal blood which can transfused to any blood type.
Analytical Skills: Takebayashi has shown to have quite good observational skills like Shiota, but, unlike the latter, Takebayashi is inclined towards statistical data and analysis as opposed to emotional and practical knowledge. His analytical abilities gives him the edge in assassination when calculating the physics and critical weak points as demonstrated during Class-E’s baseball practice where Takebayashi has been assigned to gather data on the team’s performance.
Risk Management: When some members of class 3-E got infected by an unknown virus, Takebayashi disagreed with Terasaka's suggestion of taking the infected classmates to a big city hospital because of risk factors associated with the unknown virus and very unlikely chance of an effective anti-virus. While tending some of the infected Class E members,  his observation concluded that the unknown virus was likely oral transmission over contamination.
Demolition Expert: Takebayashi is the most proficient demolition expert in Class 3-E. He easily memorizes the usage of explosives when assigned by Karasuma. He demonstrates this ability when he uses a shaped charge to infiltrate the God of Death's complex, using his smuggled explosives set to escape from captivity in Chapter 167, and distracting Houjou with flash bangs.
Academics: In the second semester finals, Taebayashi has the fifth highest overall score, at 447.
- Main article: Manami Okuda
Takebayashi and Okuda are the same type and shared the same interest, Takebayashi often teaming up with Okuda when they were still in Class E, for example when they took care of sick students together in the island, or when Takebayashi carried Okuda on his shoulders when they tricked Reaper in captivity, they also united their voices together when choosing side in civil war. It seems that they're actually familiar and often talk to each other, they just did not get enough screentime to show that. Okuda is also the one who looked the most emotional when Takebayashi left Class E in chapter 77, saying that they had fun during the regular school year too. They are later teaming up to conduct research for the RH- blood type seven years later using legacy of Korosensei. Gaining impression of the superior scientist, they start working on researching it further.
- Main article: Ryōma Terasaka
After evangelizing to him about maid cafes, they tend to meet up at around noon (to go to one). There is the vibe that Terasaka is revolving in the palm of Takebayashi’s hand.
- Main article: Gakushu Asano
Takebayashi views the Asano who excels at everything with admiration, but when comparing the way he cannot escape from his father’s bounds, he hopes that one day he can save him.
- (First line in the manga, Chapter 22) "2D is just fine...real women don't need more dimensions than that."
- (In response to Bitch-sensei, Chapter 42) Bitch-sensei: "Then what am I to all of you?"
- "Small price to pay for the old man silence. You should be grateful."
- The name Kotaro means "filial piety, obedience" (孝) (ko), "thick, big" (太) (ta) and "son" (郎) (ro).
- Kotaro's surname Takebayashi means "bamboo" (竹) (take) and "forest" (林) (hayashi/bayashi).
- His best subject is Science while his worst subject is Physical Education.
- Takebayashi is the only student who has ever decided to leave Class 3-E because of personal reasons. He was nearly deceived by Gakushu Asano and the chairman upon leaving the class. He rediscovers his motivation to kill after realizing his former classmates and Korosensei still look after him. Finally he canceled his decision and moved back to his original class.
- His favorite food is Omelette Rice.
- His past club before dropping to Class E was the Computer Club.
- In the art rankings of Volume 5, he was placed 4th among the male students.
- During the Civil War, he and Kataoka were both instantly eliminated after Chiba and Okajima sniped them.
- In the tankobon extras, his statistics were listed as:
- Physical strength: 1.5
- Mobility: 2
- Close-range assassination: 2
- Long-range assassination: 3.5
- Academic ability: 4.5
- Characteristic skill: 4.5 (trivial knowledge)
- His school rankings are:
- First Term Midterms: 87th, 350 points
- First Term Finals: 7th, 475 points
- Second Term Midterms: 14th, 446 points
- Second Term Finals: 5th, 447 points
- Assassination Classroom Manga: Chapter 20, Page 3
- Assassination Classroom Manga: Chapter 26, Page 2
- Assassination Classroom Manga: Chapter 61, Page 10
- Assassination Classroom Manga: Chapter 63, Page 3
|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|