Sakon and Ukon

and his older brother were twin brothers and the apparent leader(s) of the Sound Four. To help distinguish between the two, Sakon wore beads around his neck and Ukon had a somewhat deeper voice. Also, their bangs were on opposite sides (Ukon's being on the left, Sakon's on the right).

Personality
Sakon enjoyed bad-mouthing the opposition. Orochimaru 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 use 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.

Appearance
Both Sakon and Ukon have dark bluish-gray hair with long bangs that covered one eye. Each of the twins wore turquoise lipstick, and brown eyeliner, which made their appearance more feminine. They wore brown tunics with long black wristbands (which are mistaken being part of his sleeves), black skin-tight shorts, and purple ropes tied around their waists. Sakon wears 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.

Abilities
Sakon and Ukon are, noted by Orochimaru, to be the strongest and fastest of the entire Sound Four, surpassed only by Kimimaro. Sakon tends to prove this point by unnecessarily using extremely powerful moves to show off.

With their kekkei genkai, Soma no Ko, Sakon and Ukon were able to merge with the bodies of others. 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 blocking or attacking. Ukon can separate from Sakon's body so that the two can attack independently, though he was not used to moving on his own and thus tired easily. The two could heal quickly when merged together.

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 ogres with red skin, longer hair, and a single horn (on opposite sides for each brother). Also, as said by Ukon, they could, while in level 2, deconstruct their body on a molecular level and enter another person's body. At that point they were able to alter the cells and proteins of the target at will, effectively killing the target, 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. Also, it would appear that while one brother lays dormant in the other's body their rate of healing is increased. This generally being shown throughout the fight with Kiba as no matter what damage he did, it was seemingly negated by the pair after a quick nap in their brother's body. This seems to be a regular trait for them in videogames also.

While split apart they could use the Summoning: Rashōmon by standing at a certain distance away from each other. According to both of them this was their ultimate defense developed by Orochimaru.

Invasion of Konoha arc
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 Retrieval arc
The Sound Four escorts Sasuke Uchiha when he decides to join sides with Orochimaru. After Team Shikamaru caught up with the Sound Four, Sakon and Ukon were separated from the rest of the group by Kiba Inuzuka and Akamaru. They almost killed the two, but before they could do so they were stopped by Kankurō. Kankurō, with his puppets Karasu and Kuroari, killed Sakon and Ukon with Black Secret Technique Machine One Shot, saving Kiba and Akamaru.

Creation and Conception
Sakon was the final member of the Sound Four to be created by Masashi Kishimoto. After struggling to decide who 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

 * Sakon can be seen in the omake of Naruto Shippūden 129 with Kidōmaru, picking masks.
 * In Naruto: Ultimate Ninja 3, while playing as Sakon, holding up on the D-Pad (healing move) Sakon's name temporarily changes to Ukon.
 * Sakon's hair is shown as dark blue in the manga. In the anime it has a light bluish-gray color.