He murdered Kiriko Nishizawa, the girl he loved, in self-defense. He then murdered the couple, Tarō Kurosaki and Tsubasa Kamii, supposedly in an envious rage after what happened to Kiriko. Afterwards, he himself was murdered by Karen Kisaragi, becoming the eighth known victim.
Sōsuke has yellow eyes and short red hair styled to resemble flames. He wears the standard Hope's Peak Uniform, with the jacket tied around his waist.
Prior to the beginning of the Killing Game, Sōsuke tries to act reasonable and obey Sōshun Murasame's orders. He shows a somewhat aggressive demeanor, being quick to confront Ryota over his attempt to avenge Kotomi's death; additionally, he is one of the first students to grab a weapon once Karen Kisaragi begins the killing game.
After he is forced to kill Kiriko, he is horrified and seems to feel envious rage for Tarō and Tsubasa. He kills them violently, only to break down crying in remorse immediately afterwards.
Junko Enoshima gathered all of the Student Council and Izuru Kamukura in the old school building and forced them to kill each other. When Ryōta Someya picked up a gun, intending to avenge Kotomi Ikuta's death, Sōsuke tried to get him to put it down. Soshun Murasame ordered them to stop, and Sōsuke let go, causing Ryota to stumble backward, and become impaled on a katana, wielded by Karen Kisaragi.
When the Killing Game started, Sōsuke was one of the first to grab a weapon, taking a large spear. He fled into the hallway and was pursued by Kiriko, forcing him to stab her in self-defense. As Kiriko bled out on the floor, he was horrified about what he had done, confessing that he had loved her.
Later, Sōsuke killed both Tarō and Tsubasa as they attempted to commit a lovers' suicide, impaling the former with his spear. Seemingly fueled by envious rage for the couple after what happened to Kiriko, he screamed at Tsubasa to shut up before driving his spear through her by hitting it with Kiriko's shovel. He began crying uncontrollably after Tsubasa expired, unaware that Karen was approaching behind him with a chainsaw. She proceeded to kill him offscreen.
Kiriko was Sōsuke's friend. During the killing game, Sōsuke was forced to kill Kiriko when she attacked him out of fear and despair. Sōsuke showed much remorse over killing Kiriko, asking why she made him do it and confessing he loved her. His words imply he never told her about his feelings and it's unknown how she felt about him.
Since Sōsuke was the one who protested to Ryota killing Junko and Mukuro Ikusaba, it is likely they were friends and he may have even felt protective over him.
Sōsuke seemed to have respect for Soshun as his leader, as he listened to Soshun when he told him to stop tussling with Ryota over the gun.
- Of the participants of the First Killing Game, Sōsuke has the highest confirmed kill count (at 3).
- He strongly resembles Leon Kuwata in appearance, and has similar hair and eye colors as Kotomi Ikuta.