In his youth, he had dark hair and was missing a front tooth, wearing a simple T-shirt and sneakers.
As an adult, he had his hair short with a little cowlick on the right side and he wore a striped polo shirt and black pants.
For what little was seen of Kotaro as a child, he seemed like a relatively happy and normal boy, who had a close relationship with his mother.
However, after his mother gave him up, Kotaro developed a deep resentment towards his mother, feeling as though she selfishly abandoned him for the sake of strangers. This resentment eventually grew into a hatred for all heroes, viewing them as selfish individuals whose actions only served to harm their own families.
As a result of this hatred, he showed disdain for anything hero related and became both physically and emotionally abusive towards his own son whenever he showed interest in or even mentioned heroes around him. This is seen again when he attacked his child for finding the picture of his grandmother and condemning her memory. Ironically, he started doing the very thing he believed all heroes were guilty of and instilling his son's future hatred of heroes and society.
However, in spite of his cold attitude, he wasn't completely heartless, eventually questioning his own behavior towards his family and expressing remorse for his acts. Despite his resentment of his mother, he expressed tears when looking upon a picture of them together and recalling his childhood with her. Yet even in spite of this realization, upon seeing his son lose control of his Quirk, Kotaro immediately fell back to his abusive ways, even though his son was begging for his help. Although part of this was to stop him from spiraling out of control, this decision ultimately cost him his life.
Kotaro is the son of Nana Shimura and her unnamed husband. He was placed into foster care by his grieving mother when his father was killed. Nana also prevented her close friends from searching for him before her own death.
He later married and had a daughter named Hana Shimura and a son named Tenko Shimura (alias: Tomura Shigaraki). He became a businessman, making his fortune early on and building a large house for his family. Kotaro often punished his son for when he broke the one rule in the house, which was to not discuss anything hero related. To that end, he put his son outside until he apologized. One day, Kotaro discovered his children went into his office and confronted his son on finding the picture of his grandmother. Becoming enraged, he attacked his son and voiced his disgust of heroes and his mother's legacy. This sight was met with outrage by his family who watched in horror as this went on. Kotaro forced his son to sit outside in the dark. Later on, he stared at the picture of himself with his mother and the letter that she gave him. His wife and in-laws confronted him, as Kotaro knew he went too far and asked himself why he acted the way he did.
Later that night, Tenko lost control of his manifesting Quirk, which led to him unintentionally slaughtering his dog and later his family. Kotaro was alerted by the noise and discovered the carnage, Tenko begged his father for help while the latter was stunned by his son's acts. Tenko's Quirk started to destroy the area around them, while he tried to go to his father for help but the latter attacked him to get him to stop. Unfortunately, this triggered Tenko’s instincts and caused him to attack with full intent to kill, he touched his father's face and killed him in revenge for the pain he caused.
His lonely son would wander and go onto become the successor of All For One, the very villain who killed his mother, Nana Shimura. The only thing that remained of his father was his disembodied hands that his son wore as part of his villain costume until the latter's fight with Re-Destro where he destroyed his father's hand after overcoming his trauma.
- Kotaro shares multiple similarities with Kota Izumi.
- Both have black hair.
- Both hated their relatives for leaving them behind and their sake of being heroes, However, both of them expressed their remorse after understanding the meaning of heroes.
- Both are connected to the League of Villains, Kotaro with his mother killed by All For One and his son would become the next leader of the league, Kota with his parents killed by future member Muscular.
- Both have the word "Kota" in their names.