Sakon and Ukon

and his older brother were twin brothers from Otogakure and the apparent leader(s) of the Sound Four.

Personality
Sakon enjoyed bad-mouthing the opposition, and, like the rest of the Sound Five, he constantly referred his enemies as "trash". Orochimaru and Kabuto noted Sakon to be the fastest and strongest member of the Sound Four, traits Sakon was very proud of. When in battle, Sakon liked to prolong the fight by toying with the opponent and using unnecessarily powerful techniques. This tendency of Sakon's caused him to slow down the rest of the group whenever he got interested in an opponent. Ukon was able to counteract this flaw by being more impatient and hasty than his brother, forcing Sakon to quickly end his battles. He and Ukon also both had their own perception of morality, like several other Sound Four members. Ukon was described as greatly self-confident, and quick-tempered.

Appearance
Both Sakon and Ukon had straight, dark grey hair with long bangs that covered each different eye, their bangs were on opposite sides (Ukon's being on the left, Sakon's on the right). Each of the twins wore a green shade of lipstick, and had dark markings around their eyes giving them an androgynous appearance. They wore brown tunics with long black wristbands (which are mistaken for being part of their sleeves), black skin-tight shorts, and purple ropes tied around their waists. Sakon wore a dark-red, beaded necklace around his neck and a giant scroll behind his back. Both of the twins have a cursed seal planted on their foreheads which are covered by their bangs. At most appearances, Ukon is usually shown with a smirk on his face, and his head slightly tilted down. To further help distinguish between the two, Sakon wore beads around his neck and Ukon had a somewhat deeper voice. When reincarnated, the only changes in the twins' appearance were that their sclerae turned grey common in all the other ninja Kabuto brought back.

Abilities
Sakon and Ukon were noted by Orochimaru to be the strongest and fastest members of the entire Sound Four, surpassed only by Kimimaro. Sakon tended to prove this point by unnecessarily using extremely powerful moves to show off. In the anime, their strength was valued enough to be reincarnated by Kabuto Yakushi. Furthermore, Shikamaru stated that the duo's prowess was of jōnin-level.

They were also skilled in sealing techniques, as Sakon used the Dark Sealing Method as an additional seal to keep Sasuke confined in the box, while he was adapting to his Cursed Seal of Heaven.

Cursed Seal
Like the rest of the Sound Four, Sakon and Ukon had been given a cursed seal. When the second level of their seals were active, they resembled oni with red skin, long white hair, and a single horn (on opposite sides for each brother's foreheads). They also grew arms and legs with unique plate-like scales across them on opposite sides of their bodies.

With level two of their cursed seal, Sakon and Ukon could deconstruct their body on a molecular level and enter an opponent's body. At that point they were able to alter the cells and proteins of the target at will, effectively killing them, and then leaving the target to reconstruct their own body again. This technique, however, had the disadvantage of them taking the same damage as the body they had invaded, as seen when Kiba stabbed himself in the gut with a kunai causing Ukon to scream in pain and spit blood. When experiencing strong feelings of hatred and rage, he could draw even more power from the seal.

With their Level 2 active and when separated from each other, the two could perform Summoning: Rashōmon by standing at a certain distance away from each other. In the anime, after being reincarnated and modified, they could summon three Rashōmon gates instead of just one. They also incorporated a projectile launcher into the centre gate of the formation.

Cooperation Ninjutsu
Despite the volatile relationship between the members of the Sound Four, they were well-versed in a series of Cooperation Ninjutsu. They could use the Combination Transformation in order to pose as Orochimaru's bodyguards while also concealing their numbers. During the battle between the Third Hokage and Orochimaru, they could produce the Four Violet Flames Formation in order to prevent anyone from entering or leaving the confines of the barrier. When carrying Sasuke to Orochimaru, they used another technique, a barrier called the Four Black Fogs Formation which traps the target in a temporary death state in order to allow him to adapt to the cursed seal and make for easier transportation. In the anime, they also displayed another technique, the Space–Time Technique Formula: Underworld Turnover, that sends the souls of both the user and the target into the confines of a special barrier. They could also use the Sound Four: Summoning: Four Beasts Encirclement Formation, which brought out their individual summons to trap their opponents from all sides.

Kekkei Genkai
With their kekkei genkai, Sakon and Ukon are able to merge and separate with each other at will. Ukon generally used this ability to remain within Sakon's body where he could bring out some of his body parts to aid Sakon in battle. This made him a highly dangerous opponent in hand-to-hand combat, as in addition to defending Sakon's blind-spots, he allowed Sakon to perform three armed punches and three legged kicks. Even if the enemy could block the first strike, the other two would blindside him. In the anime, they could kick up a whirlwind by spinning their legs.

When Ukon separates from Sakon's body, the two can attack independently. However, he was not used to moving on his own for too long and thus he would tire out easily. Additionally, since Ukon is not usually the main body and hasn't had much experience being the main body, he had difficulty controlling his body when Sakon needs to heal inside Ukon's body. The two could heal any injures very quickly when merged together. Under the influence of the cursed seal, this ability was enhanced enough to allow them to merge with the bodies of others.

Konoha Crush
Sakon and Ukon made their first appearance with the rest of the Sound Four, erecting a barrier to keep others from interfering in Orochimaru's fight with the Third Hokage. They later escorted Orochimaru when he fled.

Sasuke Recovery Mission
The Sound Four escorts Sasuke Uchiha when he decides to join sides with Orochimaru. While resting along the way the Sasuke Recovery Team caught up to them and the Sound Four are caught by Shikamaru's Shadow Imitation Technique, leaving them unable to move. However, separating from his brother while no one was looking, Ukon attacked Shikamaru and broke his hold before merging back into his brother as the group take their leave with their pursuers temporary disposed of with Jirōbō and Kidōmaru dropping behind moments later at different points to further delay the Konoha ninja. With the Recovery Team makes a move for the casket holding Sasuke, Sakon and Ukon were separated from Tayuya by Kiba Inuzuka and Akamaru when a paper bomb went off and knocked them all off a cliff. With Ukon's presence made known in the fight that followed, he and Sakon assume their curse seal's second state after pounding Kiba. Kiba and Akamaru then use the Fang Wolf Fang technique to rip their opponents to pieces, only to find that the brothers separated at a result. When Kiba and Akamaru attempt a second Fang Wolf Fang, Sakon and Ukon used their Rashōmon to block them, being impressed that they made a dent in the iron wall. Sakon and Ukon then go on the offensive before Akamaru, taking a hit meant for his owner, shoots a stream of acidic pee in Sakon's eyes, blinding him on contact.

While his brother is forced to wash his eyes at a nearby river, Ukon uses their Kekkei Genkai to invade Kiba's body and destroy him at a cellular level. However, explaining the nature of the Parasite Demon Demolition Technique, Ukon is caught off guard when Kiba exploits it by stabbing himself in the stomach, forcing himself to leave the body before his opponent could kill them both. However, Kiba then threw the kunai at him and exploded some smoke bombs so he and Akamaru can escape, leaving a decoy rigged with explosions and kunai to fool Ukon. Surviving the trap, a furious Ukon fuses back into Sakon, who agrees to keep Kiba alive long enough so he and Ukon can kill him and his dog together.

Finding Kiba's jacket, Sakon gives it to Ukon so he can use the scent to sneak up on him undetected. However, Ukon's attempt to kill Kiba and Akamaru is stopped by Kankurō. Kankurō then engages Ukon with Karasu, but is caught by Sakon who attempts to merge onto him. However, Sakon learns his target is Kankurō's puppet Kuroari which impales him. In response, Ukon reaches the second stage of his cursed seal to shield Sakon from any further attacks. Sakon returned to his normal form and entered Ukon's body to heal, putting the brothers at a disadvantage due to Ukon not usually being the main body. Kankurō then has Karasu attack Ukon. Ukon dodges Karasu's attacks, unaware that he is being lead towards Kuroari. Kuroari then opens his hollow abdomen and Ukon clumsily falls and gets locked inside. As Ukon demands to be released, Kankurō uses his Black Secret Technique Machine One Shot to impale the brothers, Ukon's scream was heard within Kuroari followed by his and Sakon's blood leaking out of the puppet.

Fourth Shinobi World War: Confrontation
In the anime, Sakon and Ukon were reincarnated by Kabuto Yakushi along with their Sound Four team-mates during the Fourth Shinobi World War. They were deployed somewhere in the Land of Lightning's peninsula and fought Kiba again. Despite losing even when their body was enhanced by his summoner, Sakon and Ukon quickly reconstructed themselves and cast the Space–Time Technique Formula: Underworld Turnover to teleport themselves, their group, and their original opponents to another location. Sakon and Ukon rejoin their comrades in the Underworld Turnover barrier and they continue their battles with their enemies, switching opponents while Shikamaru comes up with an analysis. To reward them for their attempted efforts, Sakon and Ukon reveal that even their defeats will not release the barrier. Their battles rage on as the Sound Four overwhelm the Konoha shinobi with the hatred they hold for them. Taking advantage of this, Shikamaru continues to bait their hate to lure Naruto to the barrier and break his comrades free. With the souls of their most hated enemies freed, Sakon, Ukon and their comrades decide to take out their hatred on Naruto, only to be easily defeated before their souls return to the afterlife.

Legacy
Having assimilated their DNA along with the rest of the Sound Five in his experiment to create an identity for himself, Kabuto Yakushi gained access to the twins' kekkei genkai which he used to create a construct of Sakon from the base of his navel-snake that he morphs into any of the other Sound Five members or Orochimaru to utilise their abilities.

Video Games
Although Sakon is usually the playable character in video games, the Ultimate Ninja games switch to Ukon to heal, by holding up on the d-pad.

Creation and Conception
Sakon was the final member of the Sound Four to be created by Masashi Kishimoto. After struggling to decide whom Sakon should fight, Kishimoto decided to have Sakon take part in a two-on-two battle against Kiba and Akamaru. Thus he created Ukon and placed him within Sakon's body so that they could act as one person.

Trivia

 * The first kanji in Sakon's name (左) means "left", while the one in Ukon's name (右) means "right". Their names can also be a play on two sections of a nunchaku.
 * While in the second level of their cursed seal form, the brothers' faces bear a resemblance to demonic Hannya masks which are used in Noh performances.
 * Sakon can be seen in the omake of Naruto: Shippūden episode 129 with Kidōmaru, picking masks.
 * Sakon's hair is shown as dark blue in the manga. In the anime, it has a light blue-grey colour.
 * According to the databook(s):
 * Sakon's favourite phrase was:.
 * Ukon's favourite phrase was:.
 * Sakon wished to fight Sasuke Uchiha.
 * Ukon's hobby was sleeping.
 * Sakon and Ukon had completed 46 official missions in total: 20 D-rank, 15 C-rank, 8 B-rank, 2 A-rank, 1 S-rank.