Kaoru Kaidō is a second-year student at Seishun Academy, commonly known as Seigaku, and a regular member of its prestigious tennis club. He is widely recognized by his nickname, the "Viper" or "Mamushi" in Japanese, a moniker that reflects both his aggressive playing style and his snake-like demeanor on the court. Physically, Kaidō possesses a slender yet athletic build, standing at 173 centimeters tall, and is characterized by his sharp, upward-slanted eyes and slightly thick lips, which often part to release a characteristic hissing breath, sounding like "fshuuuu". A constant fixture of his appearance, both during matches and in practice, is his signature bandana, which is most often green in the anime but can vary, helping to keep his short, dark hair off his face.

At first glance, Kaidō presents a fierce and intimidating exterior. He has a short temper and can be socially awkward, leading many to perceive him as solitary and unapproachable. He particularly dislikes it when his nickname, "Viper," is used in a mocking or disrespectful manner, and he is known to react with visible irritation, especially when it comes from his rival, Takeshi Momoshiro. Despite this thorny facade, his personality has many softer layers. He possesses a surprisingly gentle and sensitive side, particularly towards animals, and is fond of playing with Ryoma Echizen's cat, Karupin. In the anime adaptation, this softness is further explored, showing that he is easily embarrassed by praise, can be moved to tears by heartfelt speeches, and harbors a secret, almost comical fear of ghosts and other supernatural phenomena.

What truly defines Kaidō, however, is his unwavering dedication and incredible work ethic. He lives by the motto "issun no mizu ni mo kansha," which reflects his belief in persistent effort and not wasting even a single drop of one's potential. His training regimen is famously brutal; he runs ten kilometers every morning, another ten after club activities, and a final ten kilometers each night, supplemented by countless shuttle runs and swing practice. This relentless conditioning is the foundation of his tennis style, which is that of a counter-puncher or "steady baseliner," designed to outlast opponents through superior stamina and mental fortitude. His coach and mentor, Sadaharu Inui, has even stated that no one can match Kaidō's fighting spirit and concentration. His primary goal is to prove himself as a top-tier player and to lead Seigaku to victory, a motivation that drives him to constantly improve.

Kaidō's most famous ability is the "Snake Shot," a powerful, looping forehand that curves sharply in mid-air, forcing opponents to run from side to side and exhausting them. This shot exploits his long arms and flexible wrists and is derived from a professional technique known as the "Buggy Whip Shot". Over the course of the series, he develops several formidable variations of this technique. These include the "Boomerang Snake," an almost impossible shot that travels around the net post and lands inside the court; the "Short Snake," a version with a very low bounce aimed at the forecourt; and the "Reverse Snake," a similar curving shot hit from his backhand side. He also masters the "Tornado Snake," an incredibly fast, straight shot with a unique gyro spin that minimizes air resistance. His development as a player is significantly shaped by his partnership with Inui. After losing his spot as a regular, Inui takes Kaidō under his wing, and together they form an exceptionally strong and strategic doubles pair often called the "Emerald Pair," complementing Inui's data tennis with Kaidō's raw power and endurance.

Beyond his relationship with Inui, Kaidō's most pivotal dynamic is with his fellow second-year regular, Takeshi Momoshiro. The two are fierce rivals who are constantly bickering and competing against one another. However, this rivalry is a productive one that pushes both players to grow. Their mutual understanding, born from their constant competition, makes them a surprisingly effective and formidable doubles team when they are forced to cooperate. He deeply respects the team's captain, Kunimitsu Tezuka, and the other upperclassmen, viewing them as standards to strive toward. His growth throughout the series is one of gradual maturity, learning to channel his fiery temper and trust in his teammates. In the manga's epilogue, Kaidō's development culminates in his appointment as the next captain of the Seigaku tennis club, with Momoshiro as his vice-captain, proving that his persistence and dedication have earned him the respect of the entire team.