Sasabe is a name several characters share, in the anime, manga, and the Special OVA episode. Most, if possibly all, are related to each other. They all share the same trait; they all happen to be ill-mannered.
Sasabe (Son)
Sasabe harassing Sakuno by hitting her with his racket.
This Sasabe is a high school tennis player of the who lives in the Tokyo area. He is known as the ace at the Kitagoe tennis club. He is introduced as being the likely winner of a local tournament which Ryoma Echizen is also registered in. Ryoma encounters him on the train and points out his lack of knowledge of tennis grips. Sasabe attempts to humiliate Ryoma in a practice match but, despite cheating, is still defeated. Sasabe taunts Ryoma into another set, which Ryoma obliges and switches to his dominant left hand. Sasabe's bullying extends to other people as well. His playstyle seems to be serve-and-volley, as he has a fast serve and net play, which was ineffective against Ryoma.
He is a high school student local bully, he has a lesser knowledge of grips, confusing the grip of the Western Grip with the Eastern Grip. He is the first person to play against Ryoma in th Prince of Tennis series, and loses 6-0. His only notable play is the 180-km/h Bullet Serve and a net play, which isn't much.
The Seigaku team encounter the father-son pair again in the anime while on a trip to the beach. Satoshi Horio had dropped the team's hotel voucher, which was picked up by Sasabe's father. Sasabe is revealed to own property near the beach and make a habit of crushing visitors to the beach in beach volleyball. Sasabe and his father challenge the "Seigaku Isonothings" for a trip to Hawaii (a ruse made up by Nanjiro Echizen to get challengers) and, again despite trying to cheat, are soundly defeated by Ryoma and Kaido Kaoru.
In fanbook 0.5 of New Prince of Tennis, it was stated this Sasabe is the oldest child in his family.
Sasabe (Father)
Father and son after losing a match to Echizen.
The father Sasabe is no different from his son, as he happens to badmouth Kato Kachiro's father at his tennis club and insult his son. However, he never listens to anyone's advice, which granted him a huge embarassing loss against Ryoma due to a muscle cramp from not warming up. He is revealed to be a counterpuncher in the anime, and was nicknamed the "Golden Retriever" due to being able to retrieve any balls like a highly-trained dog. He seems to be rich in the anime as well as a cheapskate, as he owns some form of property at the beach as well as owning a tennis resort in the OVA.
Sasabe (Prince of Tennis pilot design)
The first design for the Sasabe character, he was shown in the Prince of Tennis pilot. He was noted to have participated in the university Nationals. However, his attitude is no different than the current Sasabe, as he bullies Meino Nanako, acts high and mighty, and is willng to resort to cheating and violence. His playstyle got Ryoma to be aggressive as well, and also lost against Ryoma while begging for mercy, like the current Sasabe did during his first match against Ryoma. However, unlike the current series, the relation between Ryoma and Sasabe is softened.
Sasabe (College Student)
Only appears in the Special OVA Episode, his character design seems to be based off of the pilot design of Sasabe, as they share the same sunglasses and looks. Nicknamed "The Puppet Master of Meguro," he is much more skilled than his father and brother as well as more ruthless. His skills are (partially) thanks to former tennis pro Gondō "the Demon" being his coach, which was shown when he parried Ryoma's serve relatively easily and was able to flawlessly return his (yet weakened due to trying to adjust his serve) Twist Serve. However, he still lost to Ryoma when Ryoma upgraded his Twist Serve and finally shows part of his true strength, 6-4 (probably). His playstyle is unknown, but it is presumed to be counterpuncher , as he can aim his shots anywhere he wishes to aim to and make the opponent run and lose stamina.
He does not exist in the original series.
Junichi Sasabe
One of the several high school students in the U-17 Camp in the New Prince of Tennis, he was eliminated from the camp when he was unable to secure one of the 250 tennis balls in an elimination test. He challenged the middle school characters to a match but was instead made into a joke by Tezuka, Kite, Yagyu, Oshitari (Yushi) and Koharu by trying to challenge a middle schooler "with glasses." (Inui tried to avoid the situation) He ends up playing against Genichirō Sanada but only loses as Sanada uses "Rai" to destroy the strings on his racquet completely. It is unknown if he is related to any of the other Sasabe characters, but Takeshi Konomi definitely made his name intentional.