Birthday: November 11, 1992 (manga), 1994 (anime)
Gender: Male
Blood Type: AB
Age 16 (chapters 1-86), 17 (chapters 87-present)
Height: 170 cm
Weight: 56 kg
School: Kaimei High School
Class: 2-F (chapters 1-209), 3-C (chapters 210-present)
Likes: Illustrated encyclopedias, making designed t-shirts
Dislikes: Those who cause disorder
Affiliation: Student Council
Relatives: Haru Kirishima (biological mother)
Ryōsuke Kirishima (biological father)
Yūsuke Fujisaki (twin brother)
Doctor Tsubaki (Adoptive father)
Adoptive Mother
Adoptive Grandfather

He is the former vice-president and current president of Kaimei High School's Student Council. He is generally referred to by his last name, Tsubaki, only.

He has a strict personality when it comes to people abiding to the rules. He is also strong when fighting especially in close combat, but only displays such brutality when needed. He takes great responsibility for his actions, has a great sense of justice, and is shown to dislike foul play. Though he has a stiff attitude, he is also shown to be very caring especially towards his friends. Like his Bossun, he has inherited the will to help people. This was triggered when somebody saved him from being hassled by upperclassmen. His stiff composure can be interrupted when others teased him and when he competes with Bossun.

Tsubaki's biological parents were Haru and Ryōsuke, who were both involved in separate car-related accidents on the day of his birth. His mother's car was hit by a doctor, who immediately rushed both Haru and her friend Akane to his clinic. Haru died shortly after Tsubaki and his twin brother Yūsuke were born, and elsewhere his father died to save a child in the street. As Haru knew that her friend Akane would want to try and raise both children without the means to do so, she asked the doctor to find some way of preventing it from happening. He and his wife ended up adopting Tsubaki and raising him as their own child, without Akane knowing that twins had been born.

When he was 15, Tsubaki happened to look at medical records for his parents and discovered that they were both blood type O. As he was type AB, he realized that they couldn't be his biological parents. Additionally, on the day of his 15th birthday, he was hassled by some upperclassmen but saved by Bossun. As a result of the encounter, Tsubaki decided to change and become a stronger person. As a result of his glasses falling off during the fight, he never saw Bossun's face and neither of them seem aware that it was their first encounter.