Kengo Yonai is a minor character featured in the film Battle Royale II: Requiem. His background is defined by a harrowing past, as he is a previous winner of the Battle Royale program, a brutal government-mandated competition where students are forced to fight to the death. This experience led him to join the Wild Seven, a terrorist group formed by Shuya Nanahara to resist the oppressive Japanese government that created the program. Within the group, his designated role is simply that of a fighter.

In terms of personality, Yonai presents a dichotomy. He is described as someone who acts vicious and dangerous, likely a hardened exterior developed from his traumatic past and violent lifestyle. However, beneath this rough and intimidating facade, he is shown to be kind and gentle towards his friends and comrades within the Wild Seven. This suggests a loyalty and protective nature reserved for those he trusts. His motivations are rooted in fighting against the government that forced him and others to participate in the Battle Royale, seeking to dismantle the system and create a different future for the younger generation.

Yonai's role in the story is primarily as a supporting member of Shuya Nanahara's group. He participates in key confrontations with the government's forces. In one notable sequence, he is seen firing upon a group of students, though he stops once a comrade, Saki Sakurai, points out that the students are wearing the same explosive collars that were used in the Battle Royale, indicating a recognition of their shared victimhood. His most significant action is participating in a battle against the ATAT soldiers, the government's military force. This fight is a desperate delaying action intended to buy time for the children and surviving students to escape the conflict zone.

Regarding key relationships, Yonai is a trusted member of the Wild Seven, serving under the leadership of Shuya Nanahara, the protagonist of both the first and second films. He fights alongside other group members, including Saki Sakurai, Mitsugu Sakai, Maki Souda, Soji Kazama, and Satoshi Imakire. His bond with these comrades is the primary social connection visible in his character.

Yonai does not undergo a significant personal development arc within the film, as his role is that of a steadfast ally rather than a protagonist. His character remains consistent: a fierce fighter dedicated to his cause and loyal to his friends. Tragically, he meets his end during the battle against the ATAT soldiers. While not explicitly shown on screen, it is understood that he is killed in the firefight, as Shuya Nanahara is confirmed to be the only member of the Wild Seven who survives and escapes to Afghanistan. His notable abilities include proficiency with knives and bayonets, skills that make him a formidable close-quarters combatant and a valuable asset to his team.