Kaoru Kaidoh is a second-year student and a regular member of the Seishun Academy (Seigaku) tennis team. Physically, he stands 173 cm tall with a lean, long-limbed build that gives him an extended reach on the court. He is almost always seen wearing a bandana, often green in the anime, and has a habit of exhaling with a distinctive hissing sound, which has earned him the nicknames Viper or Mamushi. This moniker visibly irritates him, particularly when used by his rival, Takeshi Momoshiro.

Despite his intimidating and often scowling exterior, Kaidoh possesses a surprisingly sensitive and kind-hearted nature. He has a notable soft spot for animals, as seen when he tries to play with Ryoma Echizen's cat, Karupin, and even rescues a puppy by leaping into a river. He is also easily moved by emotional speeches and tends to blush when praised. In contrast to his tough demeanor, he harbors a strong fear of ghosts and supernatural phenomena, a trait that is highlighted in certain storylines.

Kaidoh is an exceptionally dedicated and rigorous trainer, often creating self-designed regimens that exceed even the demanding programs set by his mentor, Sadaharu Inui. His primary motivation is a deep-seated unwillingness to lose, fueling his relentless pursuit of self-improvement and pushing his physical limits to build unmatched stamina and mental fortitude. In the epilogue of the original manga, he is depicted as having matured into the captain of Seigaku, with his former rival Momoshiro serving as vice-captain, indicating that their intense rivalry has mellowed with time.

Within the Seigaku team, Kaidoh operates as a crucial pillar of strength. He shares a significant mentor-student relationship with Inui, who helps him refine his techniques and develop his strategic understanding of the game. They form a formidable doubles pair often referred to as the Emerald Pair. His most prominent dynamic is his fierce rivalry with Momoshiro. While the two are constantly trying to one-up each other, their deep understanding of one another’s play styles makes them an unexpectedly powerful doubles team. Kaidoh holds the team captain, Kunimitsu Tezuka, in high regard, respecting his authority and strength.

Kaidoh’s role in the story frequently places him in tense singles matches where his endurance is tested, as well as in crucial doubles matches where he must learn to cooperate with others. His development is marked by a key internal challenge from Rikkai University附属的 (Rikkai Dai's) Hiroshi Yagyuu, who questions Kaidoh’s over-reliance on his signature technique. In response, Kaidoh imposes a severe handicap on himself during a National tournament match, refusing to use his Snake shots and instructing his partner not to play, ultimately winning a two-on-one contest. This demonstrates his growth beyond simple technique into true mental resilience.

Kaidoh is a counter-puncher whose tennis style revolves around exceptional stamina and defensive retrieval, designed to exhaust opponents. His signature move is the Snake, a curving forehand shot derived from the Buggy Whip Shot. Under Inui’s guidance, he develops several advanced variations. The Boomerang Snake famously curves out of bounds before circling around the net pole and landing inside the court. The Short Snake is a version aimed to drop sharply behind the net player. The Reverse Snake curves in the opposite direction of a standard Snake to counter anticipation, and the Tornado Snake is a high-speed shot with intense gyro spin that reduces air resistance for more speed. He also masters a version of Yagyuu’s Laser Beam, called the Gyro Laser, which he can execute with the same form as his Tornado Snake to confuse opponents.