Sakon and Ukon

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

Personality
Described as spiteful and confident, Sakon enjoyed fighting and likes bad-mouthing the opposition. 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 to show off. This tendency of Sakon's caused him to slow down the rest of the group whenever he got interested in an opponent. Likewise, Ukon described as greatly self-confident, but was more quick-tempered than Sakon. The twins shared a good relationship, but are capable of viewing each other objectively during combat since their personalities differed from one another. Despite Sakon being more active than Ukon and was usually the main body, Sakon acknowledges his older brother's authority and faithfully obeys any orders Ukon gives him.

Like the rest of the Sound Five, the twins were extremely arrogant and constantly referred their enemies as "trash". Sakon was noted to have the worst temper among the Sound Four, and was the type that won't stop fighting until he kills his enemy. Although Sakon likes to drag out his fights, Ukon was able to counteract this flaw by being more impatient and hasty than his brother, forcing Sakon to quickly end his battles. Like some of their team-mates, Sakon and Ukon both had their own perception of morality, at least in Ukon's case as when he thought Kiba Inuzuka had abandoned his ninken partner Akamaru when escaping their battle and commented it was a heartless move.

Appearance
Both Sakon and Ukon had fair-skin and straight, dark blue hair (in the anime, it has a more of a light blue-grey colour) 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). They both wore a green shade of lipstick, and had dark markings around their eyes, giving them an androgynous appearance. In most appearances, Sakon is usually shown with a smirk on his face and his head slightly tilted down. Both of the twins have a cursed seal planted on their foreheads just above their eye, which are covered by their bangs.

The twins' attire was a black shirt under brown tunics with long sleeves and bore the symbol of the Sound Four on the hem, and black skin-tight shorts. They wore black arm-warmers (which are mistaken for being part of their sleeves), traditional black shinobi sandals with calf-length leg warmers covered by bandages, and a purple, rope-like belt tied in an inverted bow around their waists. To help distinguish between the two, Sakon wore a dark-red beaded necklace around his neck and carried a giant scroll behind his back through one of the loop of his rope-like belt, while Ukon lacks any embellishments in his uniform and had a somewhat deeper voice than his brother.

After being reincarnated, Sakon and Ukon looked like they did while alive with the exception the grey sclerae common in all the other ninja Kabuto Yakushi 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
As one of Orochimaru's closest and most powerful warriors, Sakon and Ukon had been given a cursed seal. When active in Level 1, it spread across their entire body in a pattern of curved lines and blotches. When in their Level 2 form, they resembled oni as their skin become red, gains yellow eye irises with dark black eye sclerae, and black curved markings appears from the corners of their eyes a upward fashion. Their fingernails becomes longer and sharp, lips turns black and their mouth is torn from ear to ear and gains canine teeth. They gain long, waist-length white hair, their chins elongated, and a single long horn grows on opposite sides for each brother's foreheads. Symmetrically on opposite sides of their bodies, their hardened skin turns black and folds like scales, appearing like spiky armor on their arms and legs.

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. However, this technique 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.

Summoning Technique
When separated from each other, Sakon and Ukon 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 with the rest of the Sound Four, erecting a barrier to keep others from interfering in Orochimaru's fight with the Third Hokage. Although Orochimaru succeeded in killing Hiruzen, his arms were crippled with necrosis, he ordered the Sound Four to flee and escort him back to Otogakure.

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
In Naruto: Ultimate Ninja Storm, Naruto Shippūden: Ultimate Ninja Storm Generations, and Naruto Shippūden: Ultimate Ninja Storm Revolution, Sakon and Ukon is a support-only character.

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 and Ukon can be seen in the omake of Naruto: Shippūden episode 129 with Kidōmaru, picking masks.
 * 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.