He's a punk who is one of the Shadow Group's members from Teimo High.
Not much is known regarding the origin or early years of Kosei Kuroki, other than the fact that he is one of the four members that make up Teimo Tech's feared Shadow Group and that he attacks his foes with his dangerous nun-chuck skills.
Kosei Kuroki, who is created by Ryuhei Tamura, has no information regarding any inspiration used to create him. He first appears in Episode 36 of Beelzebub. His Japanese voice actor is Kengo Kawanishi.
Kosei Kuroki and the rest of the Shadow Group first make their appearance within Beelzebub inside one of the maid cafes that is being held during the St. Ishiyama Academy Festival. Their leader, the skinhead Kotaro Mikagami, is shown questioning one of the maid waitresses about the pricing on one of the items posted on the menu. His meal, however, is interrupted when Kosei, Unsho Onizuka, and Takumi Dezaki arrive to notify him that Kiriya along with the other Teimo Tech thugs has made their move inside of the gym. They depart from the cafe and begin making their way towards the gym, to where their partner Reiji Kiriya and several dozen Teimo Tech delinquents have taken the student population hostage in the process of getting Kiriya his long-awaited revenge upon Oga Tatsumi. It is after Miki reveals to Kiriya about his involvement in the incident three years ago with Oga back in middle school that the feared Shadow Group from Teimo Tech makes their appearance inside the gym to help aid Kiriya in his quest for revenge. The unsuspecting Miki is blindsided by a powerful two-way attack that sends him flying across the gym floor from the impact of both of Takumi and Kosei's attacks.
Faced with multiple enemies and with the well-being of the St. Ishiyama students being held hostage on the line, Oga and Miki team up in order to deal with the members of the Shadow Group first before turning their attention upon the vengeful Kiriya. Oga decides to take on leader Kotaro Mikagami, Unsho Onizuka, and Reiji Kiriya while Miki takes care of both Kosei Kuroki and Takumi Dezaki. Charging forward at full speed, Unsho slams his broad shoulder into Oga with such force that it sends him sliding backwards across the gym floor, doing his best to defend himself against the pushing attack until he manages to bring the stunned Unsho to a stop. Puzzled, Kotaro wonders how Oga managed to stop Unsho's usually unstoppable advance, and before he knows it, Oga launches a "super" version of his fabledBurying Punch, slamming his fist into the gut of the flabbergasted Unsho with such force that it sends him flying across the gym, colliding with the stunned Kotaro and sends both of them smashing into the wooden bleachers with such force that they get stuck in them. Meanwhile, Miki faces off with Kosei and his dangerous num-chucks, and with a single strike, manages to not only shatter Kosei's weapon into pieces, he brings the shell-shocked Kosei down with a single hit. Watching in pure disbelief, the stunned Kiriya watches as the feared Shadow Group from Teimo Tech gets utterly humiliated in battle against both Oga and Miki, and before he knows it, he becomes the target of Oga's wrath.
Unlike some of the characters in Beelzebub that use powerful, almost superhuman-like attacks against their foes, Kosei Kuroki relies mainly upon his agility and his dangerous skills with his nun-chuck staff that he uses in battle to pummel his foes into submission. In his brief fight with Miki, Kosei tries to block the powerful attack that Miki launches at him, but is easily defeated when the attack shatters his staff and slams into his stomach. The impact from the single blow K.O's Kosei on the spot, sending him crashing to the floor as the horrified Takumi Dezaki watches in disbelief.