|Occupation||Former Singer and Guitarist|
|Japanese VA||Yosuke Iwazaki|
|English VA||Johnny Delacerda|
Yoshioka Seichiro (生ちろよしおか Seichiro Yoshioka) was a former singer and guitarist. He, along with his band members, raped Miho, Arisa, and Yuki, and are also indirectly responsible for Himiko's nomination for the real-life Btooom! game. However, he himself also nominated for the real-life Btooom! game.
Yoshioka is a tall young man with an average build. He has rose wood-colored hair and brown eyes. His attire consists of a black button-up shirt, black pants, and a St. James cross necklace.
In the present, he now wears a hoodie jacket and carries a machine gun out of nowhere.
Yoshioka is a heartless man, one who was unhesitant in exploiting women for his own benefit, as he will take everything he want and killing everyone who dare to get in his way, even if some of them are kids like Kosuke Kira and Kaguya.
In high school, Yoshioka befriended Himiko. Some time after his band Judgment Flash gained popularity, he invited Himiko and her friends Miho, Arisa, and Yuki to meet him and his band members at his apartment. While Himiko was retrieving snacks for them, Yoshioka and his band members drugged the three other and raped them on film. When Himiko returned, shocked at the whole incident, Yoshioka attempted to drug her. However, Himiko punched him and fled. Yoshioka, along with his band members, were later convicted of their crimes. However, these crimes would later indirectly cause Himiko's nomination for the real-life Btooom! game. 
Yoshioka is also nominated for the real-life Btooom! game in the present. Before re-encountering Himiko while she was still injured in the said island, Yoshioka somehow got a machine gun out of nowhere. 
Once Matthew Percier's crew arrived to get Ryouta and other surviving Btoom! player to escape while giving a time to get Yoshioka and Himiko as well, Ryouta tells Yoshioka to surrender himself to get out of the island quickly. Unfortunately, Yoshioka refused and hopeless from his own good, as he instead attempt to kill Ryouta and other Btooom! survivors. As Himiko fights back and pushes Yoshioka from the stairs of the abandoned factory, Yoshioka falls to a river nearby the factory behind, and finally being killed in that river by Ryouta for good and ends Himiko's nightmare. 
Equipments and skills
- Machine Gun
He was a skilled Singer and Guitarist