|BIM Type||Implosion, Cracker, Homing|
|Japanese VA||Miyuki Sawashiro|
|English VA||Chelsea McCurdy|
Kōsuke is a young boy of small stature. He has straight black hair that reaches his chin. He keeps his hair with one strand between his hair and the rest just hanging down and has deep red eyes.
Kōsuke wears a cyan hoodie with a drawstring. There is a pentagram on the front with the word "Pentagon" over it. He wears a dark blue vest over his hoodie. As the series progress, it seems that the sleeves of his hoodie have been torn off. Beige cropped pants and black and white tennis shoes complete his outfit. He wears his BIM pack on his left hip. Like the other Btooom! players, he has a green IC chip implanted in his left hand.
Kōsuke is sadistic, as evidenced by his cruelty towards his opponents and his massacre of frogs in his debut chapter. When Sōichi Natsume asks how he can't feel any empathy towards animals, his response is that he doesn't understand such value towards small animals. His lack of empathy, as well as his delinquent childhood, may be the result of living with his extremely abusive father, whom he is cowed by. However, the only person whom Kōsuke respects was his mother, and his father-figure, Togo. When Togo is killed by Yoshioka, Kōsuke went back to his insanity. It turns out behind his returning insanity after Togo died, in the same time he was nearly close to Yoko that it was revealed that Kira was possessed by the ghost of Yoko's deceased friend.  Once he has been defeated and saved by Ryouta; at the same time Matthew Percier's crew arrived to rescue Kira and the other Btooom!'s "nominated" victims to escape before the Foundation's mercenaries arrived shortly to assassinate them. Kira regains his sanity while still having a mental breakdown in regards of what happened to his father figure, Togo. 
He also does not hesitate on killing other people, unlike most players on the island.
- Ryōta Sakamoto: While initially hostile towards Ryouta, he became close friends with him as the series nears its end. He trusts Ryouta to the point that he doesn't doubt his judgment. This is especially shown when Ryouta duels Nobutaka at the end of the series.Template:CITE
Kōsuke was a victim of domestic and sexual violence by his father. He ran away from home multiple times and resorted to pickpocketing on the streets. This escalated and Kōsuke began to commit more serious crimes, which included murdering and, afterward. raping three women. He was arrested and convicted with a low sentence due to Natsume, his lawyer. His case was heavily debated and (presumably), one of the family members of the victims nominated Kōsuke, Kōsuke's father, and Natsume to the Btooom! game.
Kōsuke was also a heavy Btooom! online gamer, being nearly as good as Ryōta Sakamoto. Like Ryōta, Kōsuke was also an addict, playing 80 hours non-stop and skipped school repeatedly to do just so. His gaming addiction would often lead to his father further abusing him.
Kōsuke is first seen accompanying his father and his attorney, Natsume. His father finds a supplies case lodged in a tree and demands that Kōsuke use his Implosion BIM to bring it down. He kills his father, in self-defense, by placing one of his Implosion BIMs on his father's chest after his father sexually abuses him. This causes Natsume to run away from Kōsuke in fear. He is soon found by Ryōta Sakamoto and Kiyoshi Taira. Ryōta defeats Kōsuke in battle and takes his BIM bag and supplies case, leaving Kōsuke tied up to a tree.
However, Kōsuke manages to escape and teams up with Tomoaki Iwakura.  After a three way fight over a supplies case, Kōsuke meets with Ryōta again on a beach. Kōsuke begins to ask Ryōta for food, after which Ryōta replies that he didn't get the supplies. Kōsuke starts asking what Ryōta was doing, although knowing that Ryōta is trying to find the IC chips on the beach. Kōsuke teaches him how to find it using his radar. However, Kōsuke's real intent is actually to steal back his BIMs, after which Ryōta gives chase.
As Kōsuke is hiding from Ryōta, he realises that the BIMs he picked up belonged to Ryōta and so were unusable to Kōsuke. Ryōta manages to find Kōsuke and leaps at him, leading to both tumbling through the forest and falling into a hole. As the two are trapped in the hole, Kōsuke manages to convince Ryōta to give him back one of his Implosion BIMs and to let him stand on Ryōta's shoulders to elevate him to a suitable position to use his BIM. This however, turned out to be a ploy as he drops his BIM at Ryōta's feet and proceeds to jump out of the hole to escape. Thinking quickly, Ryōta grabs onto Kōsuke's leg and drags him back into the hole. As the two argue about turning off the BIM, the BIM explodes, but the blast radius is not big enough to engulf the two players who were close to the edge of the hole. The BIM's explosion causes a path leading downwards into the sewers, and after Ryōta's remaining BIMs accidentally drop into the sewer, Kōsuke makes his escape, going down into the sewer and picking up Ryōta's dropped BIMs.
Kōsuke is next seen by himself, wandering the forest with only two BIMs on hand. He goes against a group of four people (eventually named as Shigemasa Kusunoki, Yoko Higuchi, Heitaro Togo and Kenya Uesugi). Kōsuke is able to kill one of the members (Shigemasa) with his Implosion BIM and subsequently gains the latter's seeker BIMs, after which he attempts to attack the team's leader (Heitaro). He runs away, only to be spotted and contained by the remaining two of the team's members.
After all of that Kōsuke learns that the group seeks to keep a membership of four (because the initial participants were 32 and each needs eight chips in order to leave). By killing Shigemasa, Kōsuke made himself a suitable replacement. Kōsuke has no other choice than to team up with them.
A while after that Kōsuke (just him) goes after Ryōta and tries to beat him once and for all, but Ryōta is just more skilled with the game and manages to hit Kōsuke on his head with a stick. Kōsuke has a hole in his head because of Ryōta. He is seen crying and screaming till Tojo arrives and tells Kōsuke to man up.
After everything with Oda and Ryōta, Kōsuke is seen together with his teammates except for Tojo. They're kind of mad at him and Kōsuke decides to swallow it all. Then Tojo arrives and asks Kōsuke why he tried to take Ryōta on all by himself. Kōsuke says that it's because of Ryōta's rank in Btooom! Tojo gives him a rice cake, confusing Kōsuke who starts crying. He thinks about Tojo's words and his father beating him up.
Equipment and skills
Kira has the strength of an average teen his age.
Due to Kōsuke's experience with the online game of Btooom!, he is able to implement strategies used in-game into the real life game.
- He is the second youngest real-life Btooom! player at 14 years of age.
- He is one of the surviving people of the game alongside Ryouta Sakamoto, Emilia "Himiko" Mikogami, and Nobutaka Oda.
- Btooom! manga; Chapter 7, page 14
- Btooom! manga; Chapters 105, 106
- Btooom! manga; Chapter 8, page 15
- Btooom! manga; Chapter 7, page 29
- Btooom! manga, chapter 10, page 9
- Btooom! manga; Chapter 8, pages 6-22
- Btooom! manga; Chapter 9, page 7
- Btooom! manga; Chapter 12, page 23
- Btooom! manga; Chapter 39
- Btooom! manga; Chapter 42, pages 43-54
- Btooom! manga; Chapter 43, page 7
- Btooom! manga; Chapter 43, pages 9-11
- Btooom! manga; Chapter 43, pages 31-42
- Btooom! manga; Chapter 52, page 1
- Btooom! manga; Chapter 51
- Btooom! manga; Chapters 51 and 52