Kenny, often called Chief by his friends, serves as the technical specialist and strategic mind of the Bladebreakers team. His real name is Manabu Saien, though only a few people such as his parents ever use it, with most preferring the nickname that reflects his expertise in all aspects of Beyblading. Physically, Kenny is a young boy with messy light brown hair that often covers his eyes, blue glasses typically propped up on his head, and distinctive lightning-shaped marks on both of his cheeks. He usually wears a simple white t-shirt with a green tie and brown khaki pants.

Kenny's primary role is that of a Beyblade researcher and mechanic, a position he embraces fully as a self-described nerd who is far more interested in studying Beyblades and finding ways to improve them than in battling himself. He possesses genius-level intelligence, particularly in mathematics, science, and computer technology, and he writes his own software to analyze Beyblade statistics and performance data. His laptop contains a database referred to as Dizzy in the American release, which he uses to track parts and provide real-time analysis during battles. While he does not have a Bit-Beast of his own in the traditional sense, his knowledge and analytical abilities make him an indispensable asset to the team. He is also credited with helping create the Hard Metal Beyblades alongside Miguel and Emily.

Kenny's personality is defined by his enthusiasm for research and his loyalty to his friends. He can be somewhat insecure at times, as seen when he briefly felt jealous after Max joined the team, worrying that Tyson might forget about him. However, this insecurity gives way to genuine friendship as the three become close companions. Despite his non-combative preferences, Kenny demonstrates courage and a willingness to stand up for his beliefs, even challenging Tyson to a battle when necessary to prove a point. Outside of Beyblading, he works at his parents' noodle shop, which is one of the most successful in the city, and maintains excellent academic records as a top student.

In terms of his Beyblading abilities, Kenny battles infrequently and is generally considered weaker than most dedicated bladers, but he employs a distinctive gimmick-based style that allows him to hold his own against formidable opponents. His Beyblades are designed with jumping mechanisms large springs in the first season, a smaller spring in his Einstein blade during V-Force, and a refined hopping system in his Hopper blade during G-Revolution that allows the Beyblade to automatically leap over incoming attacks. His special attacks include the Hop Attack and the Frog Splash. Despite his limited battling experience, he has faced opponents such as Kai, Tyson, and Tala, managing to put up credible fights against them through his unorthodox strategies. Rather than achieving victory himself, Kenny often uses his battles to analyze an opponent's techniques and then transfers that knowledge to stronger bladers on his team.

Throughout the series, Kenny forms several key relationships. His friendship with Tyson is the most significant, beginning when he helped Tyson defeat Carlos in their first encounter and continuing as they become best friends. He initially felt threatened by Max's arrival but soon developed a strong bond with him as well. With Kai, Kenny has a more complicated dynamic, having lost badly to him in a qualifying match but later working alongside him as a teammate. In the second season, Kenny's parents are introduced, with his mother being somewhat pushy and overbearing while his father is more easy-going. He also shows romantic interest in several characters, including Mariah, Emily, Hilary, and particularly Ming-Ming in G-Revolution.

Kenny undergoes noticeable development across the series. In the first season, he primarily remains in a support role, handling repairs, upgrades, and data analysis while battling only twice, including a face-off against Kai at Lake Baikal that ends quickly. During V-Force, he becomes more confident, challenging Tyson to a battle after Tyson receives a Beyblade upgrade and using his hopping strategy to gain the advantage. In G-Revolution, Kenny steps further into his own as a blader, becoming an official competing member of the team after the departure of Max, Kai, and Ray. He updates his Einstein Beyblade for the tag team qualifier, battles Tala and proves strong enough to push him back, and spends considerable time as a fan of the pop star Ming-Ming. This progression from purely technical support to a more active participant represents his growth from a background figure into a more fully realized member of the team.