He acts like the older brother of the camp. Because he wants to raise the level of all the other courts, even though he's really a 1st Stringer, he kept himself on the 5th Court.
|“||Then I'll give out the Oni's special wool yarn toys!||”|
— Jūjirō Oni
Jūjirō Oni (鬼十次郎, Oni Jūjirō) is a High Schooler first shown in New Prince Of Tennis and amongst the best Japanese school tennis players shown in the series. He is known as The Gatekeeper from Hell.
Oni is seen in the U-17 camp in the 5th Court, sharing in common with Irie Kanata the fact that they are in lower courts than than their actual skill level. Despite being in 5th Court, the members of the 1st Court comment on how powerful he and Irie are who is in the 3rd Court. Afterwards, during the shuffle matches in the U-17 camp, the 5th Court defeats the 3rd Court due to Irie pretending his arm hurts and not being able to play meaning the 5th Court replace the 3rd Court making Oni a member of the 3rd Court.
He is later revealed as the original the No. 5 U-17 All Japan Junior Representative and member of the Top 10 team nicknamed the Genius 10, the reason why he did not participate in their Foreign Expedition was apparently to help strengthen the lower courts.
In a flashback we see Oni playing with orphans. He used to practice his tennis with them and he loved them very much to the point where he dressed up as Santa for Christmas and delivered presents to them. His whole reason that he wishes to be the No. 1 U-17 Representative is because the children wanted him to be it. Oni wants to see their happy faces when he becomes No. 1. 
Oni is tall and very muscular with extremely long and big dark red hair. His face seems somewhat rough and his canine teeth are also noticably more jagged than the average persons almost resembling a monster which is the literal meaning of the name Oni in Japanese.
Oni is shown to very serious and is always shown with a serious angry expression. Even while playing Oni never smiles. He appears to be a natural leader as he was in charge of the 5th Court until the Middle Schoolers arrived and reached the 5th Court. Oni was moved by the Middle Schoolers determination and put forward a Team Shuffle request.
Oni takes his opponents head on and plays in a way that eventually completely overwhelms opponents.
Despite his intimidating image in the U-17 Camp, he seems to have a soft side for children.
Two Years Prior To The Storyline
Two Years Prior To The Storyline Oni was invited to the U-17 Camp in his 1st Year of High School. Whilst there, he stood out with his abilities and faced Hōō Byōdōin who also stood out with his abilities. The two engaged in a fierce and violent match. During the match, Byōdōin demonstrates power that he "obtrained from a different dimension." Oni catches site of a pine mascot that he attached to his tennis racket and is reminded of his little brothers of his goal to become the No. 1 tennis player in Japan. He is then able to return Byōdōin's shots, appearing to turn into a Kishin (Ogre God), and the ball makes direct contact with Byōdōin's head, tearing his bandana and leaving a scar. Oni wins the game set 7-6.
For a year, Oni is undefeated as the No. 1 All-Japan player while Byōdōin trains at the camp on top of the cliff. One day, Oni decides to take it upon his responsibility to rear up the players and raise the level amongst the rest of the representatives, taking the form of the 5th Courtt's Gate Keeper until a new Generation rose up to change the tennis world.
Oni first appears with Irie Kanata and Tokugawa Kazuya angry that the lower court high schoolers were being defeated by the middle schoolers and scolds Junichi Sasabe for losing so easily to Sanada Genichiro and declares that all those who lost should leave immediately for how shameful their losses were.
His first match in the series is against Momoshiro Takeshi in a Player Shuffle which means that if the lower ranked player wins, he gets to change places with the player of the higher court. Oni Juujirou reveals his over-whelming power and crushes Momoshiro 6-0 with only 2 strings in his racket. Further showing his strength and the difference between him and the other 2nd String players.
He is then seen during the Middle Schoolers Tie-break elimination phase rallying with 5 balls with Tokugawa Kazuya and accepts the request of Tōyama Kintarō for a rally while Echizen Ryoma rallied with Tokugawa as the two super rookies interrupted their training session. At first, Tooyama pressures Oni, however Oni decides to get a little serious and completely overwhelms Kintaro as Tokugawa overwhelms Ryoma. They advise the super rookies to visit the Old Drunken Coach on the mountains.
Later on, after the middle schoolers reach the 5th Court, Oni is the only High Schooler left in the Court and decides to put forward a request for a team shuffle. Which is where if the lower team defeats the higher team, all members switch places regardless of those who won or lost.
Due to Irie Kanata pretending his arm hurts in Singles 1 against Atobe Keigo and allowing his match to end in a 6-6 draw despite being able to win, it takes the score to 2 wins, 2 losses and a draw. Which causes Oni to stop Jin Akutsu from playing and put himself forward as the player who plays the Tie-Break match. Oni is pitted against Takei Toshio who is only at the level of the other 3rd Court players. Oni crushes Takei Toshio 6-0. Afterwards, once the middle schoolers in Black jerseys arrive, Oni is among the players chosen to challenge the Top 20 U-17 players when they arrive back at the camp.
U-17 Camp Revolution
Oni along with the rest of the 2nd Stringers chosen to face the Top 10 are present at the arrival of the 2nd Stringers that manage to defeat No.s 11-20 of the 1st Stringers. Oni however, unlike the rest of the camp, is unsurprised, and was aware of the Revolution that took place. Oni can then be seen in the steam room with other candidates and then in his room taking care of his pet hamster.
Towards the end of the night, he challenges Kaji Kazena the No.5 for his badge revealing that Oni was the original No.5 of the 1st Stringers but did not want to participate in the International tournament so as to strengthen the lower courts. Oni effortlessly defeats Kaji and takes back his No.5 title.
Genius 10 ChallengeThe next day, the Top 10 are seated opposite the rest of the camp at the main courts and watch on as 2nd Stringers take turns in challenging them for the places. As Oni takes his seat next to the rest of the Top 10, the middle schoolers finally understood what Oni meant by "No. 5" during his first appearance.
Kintaro challenges Oni for no. 5 in the top 20 U-17 Japan. He wins the first game by using drop shots, much to the shock of the audience. It is noted by Oni that Kintaro's power, speed, skill, and rally strength has increased thanks to the mountain training. It was revealed that Kintaro may have hidden his true strength for the rematch. The two of them eventually rally using 5 balls at once. The high schoolers are surprised while watching this. The two of them rally up to 10 balls during their match, or at least to Choutaro, "There is only one ball but looks like 8". Kintaro returns all 10 balls. Oni compliments that Kintaro has completed his training.
As Oni continues to pressure Kintaro, he recalls memories of his past and some children that he often played with. He remembers that his goal when entering the U-17 camp was to become the No. 1 tennis player in Japan, and he begins to play more seriously. His skill from 2 years ago resurfaces, which even Byōdōin seems to respect. Irie notes that this is the first time since Oni decided to become the 5th Court Gate Keeper that he has shown his true strength. Oni appears to disappear when he hits his shots, and he wins the set 6 games to 3.
Despite the fact that Kinataro is completely exhausted as the two start the second set, the two continue to play until Kintaro is unable to stand. As everyone begins to suspect that Kintaro is unable to continue the match, Kintaro manages to stand, having achieved Pinnacle of Perfection. Kintaro comments on how the match is fun, which is why he "can't stop playing." Suddenly, Kintaro seems to regain his energy. Kintaro serves, but twice Oni fails to react to it.
Finally, Oni achieves Pinnacle of Perfection as well, and the two play to enjoy the game without holding back. Momoshiro asks if Oni had already achieved Pinnacle of Perfection prior to the match, but Fuji replies that the match with Kintaro provided him with an opportunity to rediscover the power sleeping inside him and his original goal. Yukimura notes that Kintaro has a "good expression." Oni ends the match by winning the second set 6 games to 1 and invites Kintaro to play against him again when he becomes even stronger.
When Ryoma gets disqualified, Tokugawa collapses with Oni and Irie beside him. Tokugawa wakes up in the infirmary. Oni explains what happened and that the 2nd String only managed to win the first match and nothing else. He also says that Ryoma got booted, but that Byoudouin made a face Oni had never seen him make before. He then tells Tokugawa to rest up and that Tokugawa will definitely win next time.
The next night, Byoudouin and Oni meet in an elevator and get into an argument about whose training methods are superior. When they get out, Byoudouin shows Oni a huge scar on his back that he apparently got abroad to support his claim that kindness is a weakness. Both Oni and Byodoin see that Tohno is rehabilitating and Byoudouin tells Oni to give Tohno back the No. 8 badge.
Prior to the U-17 World Cup
Oni along with the High School and Middle School First String went to the beach with the head coach. He tasks the tennis players to pick up girls. Those who failed had to try Mitsuya's Inui-Juice filled Rice Balls. Oni shouted to a nearby girl in English and failed to pick her up. He became a victim of Mitsuya's deadly riceballs.
|Type of Match||Partner||Opponent(s)||Score|
|2 Years Prior to the Storyline||None||Hōō Byōdōin||Won (7-6)|
|Player Shuffle||None||Momoshiro Takeshi||Won 6-0|
|Team Shuffle||None||Takei Toshio||Won 6-0|
|Foreign Expedition Team Challenge||None||Kaji Kazena||Won (Unknown)|
|Top 10 Challenge||None||Kintarou Tooyama||Won (6-3) (6-1)|
Playing Style and Techniques
Oni is Right-Handed and uses an All Rounder playstyle. Oni takes pride in his power and takes his opponents head-on. His style is quite reckless as he aims to make his opponent back down through feeling his power. Oni is able to use Jack Knife just like Momoshiro Takeshi and also, an even more powerful move called Black Jack Knife. In his match against Momoshiro, Oni was demonstated his ability to play with simply two strings in his racket shown his power when he used his two-stringed racquet to unleash a Black Jack Knife that rendered Momoshiro's wrists completely shot for the remainder of their match. Oni is able to effortlessly rally with 5 balls at once and to a further extent even with 10 balls at once. He was the No. 1 U-17 All-Japan player years ago, defeating Byōdōin Hōō in their lone encounter and reigned the camp undefeated for a year.
- Jack Knife (ジャックナイフ)
- Black Jack Knife
- Black Tomahawk
- 10 Ball Strike
- Kijin Aura
- Teni Muhō no Kiwami (Pinnacle of Perfection)
Speed - ?: There is nothing to criticize, but his movements covering of the left and right sides just seems a bit delayed. He may want to consider strengthening his explosiveness.
Power - ?: He's the kind of power player that trains by lifting boulders. However, that power of his Kijin that has been asleep for so long is immeasurable
Stamina - ?: He possesses an inexhaustable supply of stamina that even allows him to have such great power and speed in the match's end stage. This is the result of steady and honest training
Mental - ?: He's the type to raise his Mentality when driven into adversity. The advent of the Kijin is proof of having sharpened his concentration powers to their utmost limits.
Technique - ?: He doesn't just have power, but also excels in the technical side of the game. If he does want to improve more, It will be essential to refine his accuracy further.
Kurobe Memo: In his match against Tooyama, He showed a lively and vivid play almost mimicking that of the simple and naive boy he was playing against. He will from time to time make calls over to the children of Poka Poka Yokochou. There is no doubt that they are the source of his vitality.
- Oni wears tiger fur underwear as stated by roommate U-17 Camp Irie Kanata. 
- Oni has a pet hamster named Kaede. 
- In his first year of middle school, there was a girl from another school who always rode the same train as him. He got all fired up and wrote her a letter, but…(no reveal on what happened)
- Oni was taught to knit stuffed animals by the Director of Boka Boka Yokochou (an orphanage).
- The person Oni gets along the best with is Nakagauchi Sotomichi.
- Because his family is full of women, he is naturally quite fond of dependent children. 
- He probably likes children because of all the years he has spent away from his little sister.
- Oni is the only player shown to use only two strings in his racket in an official match.
- He is the only known character in both Prince of Tennis manga and New Prince of Tennis manga who is shown to have farted.
- Hobbies: Playing guitar, Making Stuffed Animals, aka amigurumi. (To give to children as presents)
- Committee: Public Relations Committee
- Favorite Color: Red
- Favorite Food: Ginko Nuts
- Thing he wants most: A new cage for his hamster
- Thing you're bad at: Crying Children
- Specialty outside of tennis: Making Stuffed Animals