Sakon and Ukon

and the older brother were twin brothers and the apparant leader(s) of The Sound Four (Orochimaru's bodyguards). 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 and Ukon 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.

Konoha Invasion arc
Sakon and Ukon were first seen during the Invasion of Konoha arc. There they, alongside the rest of the Sound Four: Jirobo, Kidomaru and Tayuya created a barrier around the fight between Orochimaru and the Third Hokage that prevented anyone from leaving and any of the ANBU from entering.

Sasuke Retrieval arc
He made his next appearance when Orochimaru ordered the Sound Four to go to Konoha to fetch Sasuke Uchiha. After arriving they saw that Sasuke was with Kakashi Hatake and decided to wait a little bit before approaching him. After Kakashi left they approached Sasuke which in turn resulted in a fight that Sasuke, at first, seemed to be winning.

However when Sakon and Ukon activated their cursed seal, they became too powerful for Sasuke to deal with and he was left with the offer of following them to Orochimaru who would make him stronger.

During the Sasuke Retrieval arc, Sakon and Ukon fought and almost killed Kiba Inuzuka and Akamaru. Before they could finish the two off, however, Kankuro with his puppets Karasu and Kuroari arrived and killed them both, saving Kiba and Akamaru.

Abillities
With their kekkei genkai, 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.

Like the rest of the Sound Four, Sakon and Ukon had been given Orochimaru's cursed seal. Both had the same type of cursed seal, which consisted of three tomoe which seemed to be hidden under his hair on the right side of his forehead for Sakon and under the left side for Ukon. When spreading it resembled a bacterial infection.

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, he could, while in level 2, deconstruct his body on a molecular level and enter another person's body. At that point he was able to alter the cells and proteins of his target at will, effectively killing them, and then leaving them to reconstruct his own body again. This technique, however, had the disadvantage of him taking the same damage as the body he has invaded, as seen when Kiba stabbed himself in the stomach with a Kunai.

However this ability also allowed them to regenerate faster, though at the cost of a lot of chakra Also, while in the second stage of their cursed seal form, Sakon and Ukon could combine their bodies, allowing them both to act at the same time and have both of their heads appear, though it would seem that it was still only Sakon who was in control.

Also if they were hit by a strong attack they could split up their bodies and grow out their missing arm and leg in the form of an armored flesh. Also while split apart can they use the Summoning: Rashomon by standing at a certain distance away from each other and then each using the summoning technique, to summon a large gate that has demonic face like features on it. According to both of them this was their ultimate defense developed by Orochimaru.

An interesting fact is that their Level Two forms are so far the only ones that bare almost no resemblance to their previous forms.

While it isn’t noticeable, Sakon also displayed the ability to manipulate sound with his voice in the form of musical notes during the small fight that escalated between the Sound Four and Sasuke Uchiha.

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 and Ukon have a multiple personality just like Zetsu.