|Occupation||Former Singer and Guitarist|
|Japanese VA||Yousuke Iwazaki|
|English VA||Johnny Delacerda|
Seichiro Yoshioka (吉岡 清志郎 Yoshioka Seichirō) 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 is 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 wants and killing everyone who dares 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 others 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 for 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. At the first day of the game, he allied with Heitaro Togo, Yoko Higuchi, and Suzume Yosano. When he and Yosano were looking around the place, she betrayed Yoshioka and attempted to kill him with a homing BIM. Yoshioka fell down from the cliff and landed on the water. After Togo killed Yosano, he tried to do CPR on Yoshioka, but he didn't regain consciousness and his heart stopped, dropping the IC chip. However, he was somehow alive despite the fact that his heart stopped and woke up. 
He joined the sanctuary group for a short time before leaving the group because he was annoyed with the rules of the group. Before re-encountering Himiko while she was still injured in the said island, Yoshioka somehow managed to get a machine gun and plenty of ammunitions for it,  most likely from scavenging the remains of soldiers who arrived on the island.
Once Matthew Percier's crew arrived to get Ryouta and the other surviving Btoom! players to escape while giving 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 for his own good, he instead attempted to kill Ryouta and the 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 is finally killed in the river by Ryouta for good which ends Himiko's nightmare. 
Equipments and skills
- Machine Gun
- Btooom! anime episode 2
- Btooom! manga chapter 93 page 24
- Btooom! manga chapter 90
- Btooom! manga chapter 106