Sakon and Ukon

and the older brother are twin brothers and the apparant leader(s) of The Sound Four (Orochimaru's bodyguards). To help distinguish between the two, Sakon wears beads around his neck and Ukon has a somewhat deeper voice. Also, their bangs are on opposite sides (Ukon's being on the left, Sakon's on the right).

Personality
Sakon and Ukon enjoy bad-mouthing the opposition. Orochimaru notes Sakon to be the fastest and strongest member of the Sound Four, traits Sakon is very proud of. When in battle Sakon likes to prolong the fight by toying with the opponent and use unnecessarily powerful techniques. This tendency of Sakon's causes him to slow down the rest of the group whenever he gets interested in an opponent. Ukon is 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 are first seen during the Invasion of Konoha arc. There they, alongside the rest of the Sound Four: Jirobo, Kidomaru and Tayuya creates a barrier around the fight between Orochimaru and the Third Hokage that prevents anyone from leaving and any of the ANBU from entering.

Sasuke Retrieval arc
He makes his next appearance when Orochimaru orders the Sound Four to go to Konoha to fetch Sasuke Uchiha. After arriving they see that Sasuke is with Kakashi Hatake and decide to wait a little bit before approaching him. After Kakashi leaves they approach Sasuke which in turn results in a fight that Sasuke, at first, seems to be winning.

However when Sakon and Ukon activate their cursed seal, they become too powerful for Sasuke to deal with and he is left with the offer of following them to Orochimaru who will make him stronger.

During the Sasuke Retrieval arc, Sakon and Ukon fight and almost kill Kiba Inuzuka and Akamaru. Before they can finish the two off, however, Kankuro with his puppets Karasu and Kuroari arrive and kill them both, saving Kiba and Akamaru.

Abillities
With their kekkei genkai, Sakon and Ukon are able to merge with the bodies of others. Ukon generally uses this ability to remain within Sakon's body where he can 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 is not used to moving on his own and thus tires easily.

Like the rest of the Sound Four has Sakon and Ukon been given Orochimaru's cursed seal. Both have the same type of cursed seal, which consists of three tomoe which seems 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 resembles a bacterial infection.

When the second level of their seals are active, they resemble ogres with red skin, longer hair, and a single horn (on opposite sides for each brother). Also, as said by Ukon, he can, while in level 2, deconstruct his body on a molecular level and enter another person's body. At that point he is 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, has 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 allows 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 can 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 is still only Sakon who is in control.

Also if they are hit by a strong attack they can 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 is this 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.

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.