Kaidou Kaoru

Kaidoh's nickname is "Mamushi", or the "Viper," which fits him well. He wears a bandana (in the anime, it is orange until episode 12 when it changed permanently to green) and often exhales making a "fshuuuu" sound, similar to a snake. Kaidoh does not like this nickname, and often reacts badly when called that within earshot, especially if the person uttering the nickname is Takeshi Momoshiro or Akira Kamio (he once even comes to try hitting Kamio during the Fudomine matches, placing Tezuka in a rather uncomfortable situation.)
His signature move is the "Snake Shot" (derived from the "Buggy Whip Shot"), 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 developed the shot under mentor Sadaharu Inui's advice and by working with him in doubles. Kaidoh trains rigorously, increasing the training program given to him by Inui to give him unbeatable endurance. Both he and Takeshi Momoshiro are second year regulars and rivals. They constantly try to one-up each other, which is actually beneficial for the growth of their tennis skills; this makes them extremely powerful as a doubles pair, because the two rivals know each other so well.
Kaidoh and Inui also make an excellent doubles pair, mostly after beating Inui himself during the same line-up matches where Ryoma became a regular. Inui has seen Kaidoh's potential and kind side and has taken him under his wing. In the fandom, they are often known as the "Emerald Pair." This is presumably a pun based on the name of Seigaku's "Golden Pair" (Eiji Kikumaru and Shuichiro Oishi) and several green items associated with the Emerald Pair, such as Kaidoh's bandana, Inui's notebook, Inui's special juices and Inui's pants he used when being the "coach" of Seigaku after losing his regular place to Ryoma Echizen.
In Genius 379, the last chapter of the manga, Kaidoh is seen as a third year and the new captain of Seigaku, with Momoshiro as his sub-captain. Their rivalry has, apparently, somewhat mellowed with time.

Takeshi Momoshiro

Momoshiro is known as Seigaku’s number one rascal. He really wants to be liked by everyone, which is evident by his preference to be called "Momo-chan," even by the freshmen. As a compromise, some of the freshman call him "Momo-chan-senpai" while the third years call him by "Momo." Momoshiro is also very similar to the American former pro Pete Sampras.
At the beginning of the series, he played an unfinished match with Ryoma Echizen (he did this to see the twist serve), though Momoshiro was injured at the time. Later, they become best friends and even doubles partners for once. Momoshiro's signature move is the "Dunk Smash," a very powerful response to lobs as he leaps several feet into the air, which he likes to finish off saying, "Don," upon landing. He later develops the "Jack Knife," a strong backhand shot.
Momoshiro is an aggressive baseliner. For a short time, he is cut from the regulars, after losing to Inui and Tezuka during Seigaku's 2nd monthly ranking matches. He takes being cut pretty hard and doesn't show up to practices for three days, leading the first years to think that Momoshiro has quit the club. Instead of going to practice, he spent his time playing street tennis; however, as he plays a match with An Tachibana, a good friend of his, he realizes his flaws as a tennis player and that in order for him to become stronger, he needs to stop underestimating his opponents. It is during this time that Momoshiro's importance to Seigaku as a team is revealed.
As the player who is the most open and friendly with everyone, Momoshiro is connected to everyone on the team on some level. And as such, he's one of the few members on the team that everyone seems to care about, which is why his disappearance affects all the regulars in some way. Momoshiro being cut from the team even leads to the temporary break up of the Golden Pair and even Eiji notes how practices are not the same without Momoshiro around. Although Ryoma, in particular, seems unconcerned about Momoshiro being cut from the team, in reality, he is worried about Momoshiro and goes looking for him during lunch period; a moment that reveals both how important Momoshiro's friendship is to Ryoma and how important Momoshiro is to Seigaku. Thus, when Momoshiro returns to practices, all seems right with the team again, although Tezuka punishes him for missing practice by making him run a record high of 100 laps. This is also when Momoshiro as a character is revealed in that Momoshiro doesn't worry about taking himself too seriously and despite being cut from the team, he is determined to support and cheer his teammates on as they face Hyotei in the Regionals.

Sadaharu Inui

Four years prior to the start of the series, Inui and Renji Yanagi from Rikkaidai were an unstoppable Elementary School Doubles Pairing. Unfortunately, their doubles pairing was ended when Yanagi suddenly moved to the Kanagawa Prefecture.
Inui is a highly intelligent player. He plays what he calls "Data Tennis," which involves collecting and analysing data for a player in order to determine weaknesses. He is even able to determine the exact probability of certain shots being successful. The Data Tennis style actually originated from Yanagi. To his surprise, Data Tennis does not work against Ryoma Echizen who evolves over the course of a match, and Jin Akutsu (although Inui never played against him), due to his natural flexibility making shots unpredictable. Inui also finds it difficult to get accurate data on Shusuke Fuji and Kunimitsu Tezuka.
Inui was beaten by Kaido at the beginning of the series and became the team's manager. He uses his data to create exercise programs for the team. His most hated innovation would have to be his "Inui Juice" or "Penal Tea" (penalty), a disgusting health drink which is used as punishment. The contents of "Inui Juice" are unknown, but Inui is always coming up with new and "better" recipes. When the team went bowling, Inui's new invention "Aozu" first knocked out Kaido and even Fuji (who usually enjoyed drinking the "Inui Juice" or "Penal Tea") as well as the rest of the team. Even Inui himself falls victim to the "Aozu".

Takashi Kawamura

Kawamura is a soft-spoken person off the court, very gentle and shy, though still outspoken and stubborn when he takes a decision. However, he becomes extremely aggressive and loud once he grabs his racket, or any racket. This change in his personality is marked by him shouting "Burning!!!"
Kawamura's style of play is aggressive baseliner and in the original Japanese, he often punctuates his play with English expressions (In the English adaptations, he uses rather unorthodox language). There was a match were he was especially excited and the three first years commented that he was amazing, but that he was no longer speaking Japanese.Kawamura joined Seigaku's tennis club as a first year student. Although he had a lot of power, he was ridiculed by many of the older members of the club because he had no other skills, and couldn't make successful shots; the sempais even nicknamed him "home-run guy" and said he should switch to the baseball team. However, he found close friends in his fellow first years, most of whom were notoriously skilled, such as Kunimitsu Tezuka, Shusuke Fuji, Shuichiro Oishi, Eiji Kikumaru, and Sadaharu Inui; Tezuka even told Kawamura openly that he was envious of his physical power. Together with them, he became a part of Seigaku's future.

Eiji Kikumaru

Eiji Kikumaru is a third year student at Seishun Academy (Seigaku). He is in the same class as Shusuke Fuji and also happens to be one of his good friends.
He is part of Seigaku's "Golden Pair" along with Shuichiro Oishi. Using his acrobatic play he can return almost any ball, even those that seem out of human reach. Eiji is also known to have the sharpest vision on the team, with an exceptional talent for seeing moving objects. The only other member of his team to have this talent is Ryoma Echizen. This proves to be both a strength and a weakness, especially when matched against St. Rudolph's team captain Akazawa. Other than his well known style of acrobatic play, he has his own signature smash move called " Kikumaru Beam;" which is essentially a very high behind-the-back smash that is launched either at the corner of the opponent's court, or directly at the opponent.
Though the Golden Pair seldom play without each other, Eiji has been paired with his friend Shusuke Fuji, Seigaku's genius, to form the Dream Pair, as well as forming a makeshift pair with Takeshi Momoshiro. In the Nationals story arc of the manga, Eiji plays singles after Oishi gave his spot to Echizen. He says that if Oishi will not be his partner, that he refuses to play doubles anymore. After his first match against Higa Middle School, however, he proclaims that playing singles tennis is "too lonely," and the Golden Pair becomes a team again.
Eiji's main weakness is a lack of stamina, which he overcomes through hard work and training later on in the series. His main rival is Hyotei Gakuen's Gakuto Mukahi, another acrobatic player who is extremely light and surpasses him in acrobatic play.He is a friendly, sweet-tempered and outgoing character and the youngest of a large family. So unlike most of the characters, Eiji is usually referred to by his given name.
Eiji acts like a cat, which shows in his acrobatics and how he likes to add a "Nya" (the Japanese onomatopoeia for a cat's meow) at the end of his sentences. This is especially evident in the chibi episodes where in the TeniPuri Family stories he is the family's pet cat.

Shusuke Fuji

Shusuke Fuji is a fictional character in the anime and manga The Prince of Tennis. His name is romanized as Syusuke Fuji in the Japanese anime, and Shusuke Fuji in all the English versions.
The kanji for Fuji stands for 'not second', 'unparalleled', or 'peerless'. His name can also be seen as a bit of a pun when his hobbies and favorite foods are taken into account; Fujifilm is the world's largest photographic and imaging company, and one of Fuji's hobbies is photography. Fuji apples are a type of apple, and they are one of Fuji's favorite foods.
In the Valentine Chocolate specials from the manga, Fuji has constantly been in the top three recipients of chocolate, with the exception of the years 2005 and 2006.Shusuke Fuji is a third year student at Seishun Academy, and is approaching fifteen years of age at the beginning of the series. He is given the title "tensai", or genius, due to his tactical skill on the tennis court. He is in the same class as Eiji Kikumaru, and is Eiji's good friend. Fuji is a leap year baby (his birthday is on February 29), and thus he can only properly celebrate his birthday once every four years. Fuji is cheerful and easygoing, with a penchant for playfully teasing those around him.

Shuichiro Oishi

Shuichiro Oishi is a fictional character in the anime and manga The Prince of Tennis. His name is spelled Shuichiro Oishi in the English manga, and it is spelled as Syuichirou Oishi in Roman characters in the Japanese version of the TV show. It is spelled "Syuitiroh" in some artwork, probably because the "chi" hiragana technically belongs to the "t" row.
In the fourth character popularity poll for the series run in Weekly Shonen Jump, Oishi came in 8th place.Oishi is a third year student at Seishun Academy (Seigaku) and the co-captain of the tennis club. Along with Eiji Kikumaru he is part of Seigaku's "Golden Pair," a doubles team that made it to Nationals the year before the series takes place. His only special shot is the "Moon Volley." This shot is a topspin lob that travels very high and lands just inbounds. Oishi is calm and rational in a match, and while he is not the greatest of singles players, as a doubles partner he is the perfect complement for the acrobatic Eiji.