Sasuke Tsubaki was born to Haru and Ryōsuke Tsubaki, both of whom were involved in separate automobile accidents on the day of his birth. His mother’s car collided with that of a doctor, who immediately transported Haru and her friend Akane to his clinic. Haru gave birth to twin sons, Sasuke and Yūsuke, but died shortly afterward. On the same day, Ryōsuke was killed while saving a child in the street.

Before her death, Haru expressed concern that Akane might try to raise both infants despite lacking the means to do so. To honor her wishes, the attending doctor and his wife secretly adopted Sasuke and raised him as their own son, concealing from Akane the fact that twins had been born.

At the age of fifteen, Tsubaki discovered medical records revealing that both of his adoptive parents had blood type O, while his own blood type was AB, an impossibility genetically. This realization led him to understand that he was adopted. On his fifteenth birthday, he was confronted by upperclassmen but was rescued by a boy later revealed to be Bossun, his twin brother Yūsuke. The two did not recognize each other at the time, as Tsubaki lost his glasses during the encounter and never saw Bossun’s face. The event inspired Tsubaki to change himself and strive to become a stronger and more disciplined individual.

Sasuke Tsubaki bears a strong resemblance to his twin brother, Bossun, sharing the same dark, messy hair, though his is kept shorter and neater. He is almost always seen wearing the standard Kaimei High School uniform, which he maintains in an orderly fashion, reflecting his disciplined personality. As vice-president of the Student Council, he wears an identifying armband on his left arm. One of his most distinct physical features is his long eyelashes, a trait believed to be inherited from his biological mother, Haru.

In flashbacks to two years before the main events of the series, Tsubaki is shown with longer hair, similar in length to Bossun’s, and he wears glasses. This earlier appearance corresponds to the time when he was saved by Bossun, an encounter that neither brother recognized as their first meeting.

Sasuke Tsubaki is characterized by his disciplined and rule-abiding nature. As a student council leader, he upholds order and expects others to adhere to school regulations, often coming across as strict or inflexible. Despite his stern demeanor, he is deeply responsible and guided by a strong sense of justice, showing little tolerance for dishonesty or foul play.

Tsubaki is also a capable fighter, particularly skilled in close combat, though he only resorts to physical confrontation when absolutely necessary. Beneath his serious exterior, he is caring and protective toward his friends and subordinates, demonstrating genuine concern for their well-being. Much like his twin brother, Bossun, he possesses a natural desire to help others, a trait that was awakened after being rescued from upperclassmen as a teenager. His otherwise composed personality tends to falter when he is teased or when his competitive streak surfaces during interactions with Bossun, revealing a more emotional and human side beneath his rigidity.

Martial Arts
Sasuke Tsubaki holds black belts in both karate and judo, demonstrating advanced proficiency in close-quarters combat. His fighting style emphasizes powerful punching techniques, though he also possesses training in other martial disciplines, including kendo. Upon entering Kaimei High School, his skill drew the attention of the boxing club, which attempted to recruit him. Tsubaki’s physical strength is considerable—his punches are strong enough to leave visible damage on solid surfaces, as shown during his first encounter with Shinzō.

Sōjirō Agata
Tsubaki greatly admires and respects Sōjirō Agata, continuing to address him as “President” even after Agata’s graduation. Although they occasionally differ in opinion, Tsubaki remains receptive to Agata’s advice and requests. Agata, in turn, holds Tsubaki in high regard, once remarking that he would be comfortable entrusting his sister, Saaya, to Tsubaki’s care should she develop feelings for him.

Bossun (Yūsuke Fujisaki)
Tsubaki’s twin brother, Bossun, serves as both his rival and his mirror. The two frequently argue and compete, yet their interactions also reveal moments of genuine brotherly affection. Unbeknownst to either, their first encounter occurred on their fifteenth birthday, when Bossun rescued Tsubaki from a group of bullies, an event that inspired Tsubaki’s drive to become stronger. Their contrasting personalities define much of their dynamic: Tsubaki is strict and physically strong, while Bossun is adaptable and emotionally intuitive. Despite these differences, both share a fundamental desire to help others. Over time, their relationship matures into one of mutual respect, exemplified when they cooperate to stop a thief. When Agata asks Tsubaki how he envisions their future, Tsubaki admits he wishes to be closer to his brother, though their stubbornness often prevents it.

Kiri Katō
Initially, Kiri Katō dismisses Tsubaki’s authority as student council president and refuses to follow his leadership. However, after the incident involving Kutsuwa, during which Tsubaki risked himself rather than abandon Katō, Katō becomes one of Tsubaki’s most loyal allies and self-appointed bodyguard. Their relationship is often compared to that of Bossun and Himeko, with Tsubaki occasionally expressing frustration at Katō’s overprotectiveness. Despite Tsubaki’s occasional orders for Katō to stop shadowing him, this remains one command Katō consistently ignores, reflecting his unwavering loyalty.