Kaidoh Kaoru is a second-year student at Seishun Gakuen, commonly known as Seigaku, where he is a regular member of the tennis club. He is recognizable by his lean build, standing 173 centimeters tall, and his almost constant accessory: a headband, most often depicted with a green and white striped pattern. His most distinctive habit is a sharp, hissing exhale that sounds like "fshuuuu," which has earned him the nickname "Mamushi," meaning viper. He dislikes this name when it is used in a mocking manner, particularly by his rival, Momoshiro Takeshi.

Outwardly, Kaidoh projects a stern and intimidating presence. He has a short temper, a fiercely competitive nature, and often wears a scowl, with his large, upward-angled eyes contributing to his intense glare. Despite this fearsome exterior, his personality is more nuanced. He is, in fact, deeply sensitive and can be easily moved to tears or blushing when he receives praise. He possesses a gentle side that is most apparent in his affinity for small animals, as he is often shown trying to play with Echizen Ryoma's cat, Karupin. This inner softness contrasts sharply with his aggressive on-court persona.

Kaidoh was born on May 11th. He has a strong sense of discipline and lives by the motto "Constant dripping wears away the stone." This philosophy is evident in his daily routine, which includes a grueling self-imposed training regimen of long-distance running, often covering twenty-five kilometers or more each day. He is a student of few words, and his speech is often punctuated by his signature "fshuuuu" or exclamations like "Don't underestimate me!".

His primary motivation as an athlete is to achieve victory through relentless persistence and mental fortitude. His playing style is that of a counter-puncher, defined not by flashy net play but by extraordinary stamina. He excels at dragging opponents into long, grueling baseline rallies designed to test their will and physical endurance. His singular goal is to improve his own tennis, honing his body into a weapon that can outlast any challenger.

Within the story, Kaidoh serves as a powerful representative of the "hard work" ethic to complement the "genius" displayed by other characters. He is a staple in the singles lineup for Seigaku, trusted by the team captain to secure wins through sheer determination. His role is to push himself to the absolute limit and, in doing so, push his opponents past theirs. He often serves as the team's anchor in high-pressure situations.

Kaidoh's most significant relationships are defined by rivalry and mentorship. His most famous dynamic is with Momoshiro Takeshi, his fellow second-year regular. They are fierce rivals who constantly bicker and compete, yet their mutual respect and profound understanding of each other's tennis make them an exceptionally effective doubles pair when they set aside their differences. He also shares a deep bond with the third-year student, Inui Sadaharu. Inui acts as a mentor, using data analysis to help Kaidoh refine his techniques. The two form the "Emerald Pair" in doubles, where Kaidoh's physical power combines with Inui's strategic mind. He also holds great respect for the team captain, Tezuka Kunimitsu, as the epitome of a strong and dedicated leader.

Over the course of the series, Kaidoh undergoes significant development, primarily in his tennis abilities. He starts with his signature shot, the Snake, a powerful looping shot that curves sharply from one side of the court to the other. To overcome his limitations, he develops several advanced variations. The Boomerang Snake is a shot that curves so dramatically it travels around the net post and still lands inside the court. He later creates the Short Snake for doubles play, the Reverse Snake to counter specific opponents, and the Tornado Snake, an extremely fast, gyro-spinning shot that minimizes air resistance. He even masters a version of the Laser Beam. This evolution shows his adaptability and willingness to step outside his comfort zone to grow. His most notable development, however, is his ability to occasionally forgo his own strengths entirely, as seen when he won a doubles match without using any Snake shots at all, proving his growth is not just technical but also tactical and mental.

Kaidoh's notable abilities are almost entirely focused on his unparalleled conditioning and his deceptive shot-making. His signature Snake series relies on an extreme upward swing that puts a massive amount of topspin on the ball, causing its trajectory to bend unnaturally. This forces opponents to run from sideline to sideline, draining their energy. The advanced versions of this shot add elements of surprise, such as drastically different spin or altered speed, making them nearly impossible to anticipate. His physical data, including an exceptional score in the twenty-meter shuttle run, confirms his status as one of the most physically fit players in the series.