Kaoru Kaidoh (海堂 薫, Kaidō Kaoru?), known as "Viper" to his teammates, is known for constantly making a "fshuuuu", sound, similar to a snake, while exhaling. Kaidoh trains rigorously, increasing the training program given to him by Inui to give him unbeatable endurance. He has a deep rivalry with Momoshiro, in which they constantly try to one-up each other. He normally plays singles, though he eventually pairs with Inui, who helps Kaido train at first. Kaido's stamina is used to let him play the other team alone, while Inui collects data. After the national tournament, Kaidoh becomes the new captain of Seigaku. He is voiced by Kōhei Kiyasu in Japanese media, voiced by David Lodge in English media, and portrayed by Kousuke Kujirai in the live-action The Prince of Tennis film.

His signature move is the "Snake Shot", derived from the "Buggy Whip Shot", which is a curving forehand that keeps his opponents running. He later develops the "Boomerang Snake," which is a shot that swerves out of bounds, around the net pole, and back in bounds. He develops the shot under Inui's advice and by working with him in doubles. He also develops the "Short Snake", which goes directly behind the net player, the "Reverse Snake", which is used to keep opponents guessing, and the "Tornado Snake", which is a fast straight shot with a spin that cuts air resistance. He can also use Hiroshi Yagyuu's "Laser Beam", calling his version the "Gyro Laser", which is an extremely fast shot with the same spin as the Tornado Snake, allowing him to confuse opponents. He also develops the "Hadoukyuu boomerang" which is developed from the hadoukyuu that fudoumine, Ishida, made, which he uses a one handed hadoukyuu, and so kaido uses ishida's hadoukyuu and his boomerang, and puts it together to perform a very powerful shot that is just like a regular "boomerang" but it is much more powerful.

Although he has a tough exterior and is cold towards others, there is a softer side to him. Unknown to his teammates, or anyone else in general, Kaidoh enjoys small animals, often taking the time to play with them. There are oftentimes shown in the series where he is seen playing with Ryoma's cat, Karupin, which wanders off on its own most of the time. It also seems as if Kaidoh is afraid of anything related to the supernatural. This is shown in episode 99.

Despite being interpreted as a tough teenager throughout the entire series, Kaidoh is actually a very kind person. In episode 72, three kids mistake Kaidoh for Ryoma, ( due to a misprint in a magazine ) so throughout that episode, he tries his best to mimic the first - year as best as he possible can. He mainly just embarrasses himself to a certain point where he snapped, going into a rage and sending out a flurry of his own tennis moves, claiming that he was "Kaidoh, Kaoru!" ( All of this happened while playing against Momoshiro, who teased him to that point. ) Kaidoh does this ( pretending to be Ryoma ) in hopes to not disappoint one of the kids, who looked up to "Ryoma", and believed that Ryoma was better than his friend's neighbor. ( Which was revealed to be Momoshiro. ) After realizing what he had done, Kaidoh turns to the kids thinking he really messed up, though little did he know; that day he gained four fans. ( The real Ryoma claimed to be, "Horio with two years of tennis experience." )

In the various TeniPuri Family episodes, Kaidoh is the young infant. Not being able to speak, he mainly just goes: "fssshhuuuu". Shuuko ( Oishi ) introduced infant - Kaidoh as "cutie-pie, Kaoru-chan". He is often bullied in some sorts by his older siblings, Ryoma and Momoko ( Momoshiro ) .