Zetsu

is a member of Akatsuki functioning as the organization's spy.

Background
Very little is known about Zetsu's history before his time in the Akatsuki. So far, there have been no flashbacks or mentions about his village of origin or his crime that led to his defection, or if he even is a missing-nin in the first place. Many fans believe that he is from Kusagakure, but that has yet to have been made official.

Personality
Zetsu is easily distinguished by two large venus fly trap-like extensions enveloping his head and upper body. These extensions can envelope the rest of his body as well, giving him a plant-like appearance. When his extensions are open and his head is visible, it is revealed Zetsu has two different colored halves of his body, the left side completely white and the right side black. To go with this, Zetsu seems to have a split personality, as in the Japanese version of the manga the black half of his face speaks only in Katakana while the white side uses Kanji and Kana, and in the English episodes the black side has a different voice actor than the white side. The two halves also seem to converse with each other, and occasionally differ in opinion (the black half is more serious and intelligent while the white half is carefree and easygoing). They also seem to communicate with their thoughts. To further his venus fly trap theme, Zetsu is also a cannibal.

Zetsu's main role within the Akatsuki is to function as a spy, often acting as a hunter-nin for Akatsuki. He has been shown able to move his body and take part in the tailed beasts sealing ritual simultaneously which, in addition to being unique to him, allows him to act as a lookout while the sealing takes place. This might refer to the split personality theme; since he acts as two different people, one side might focus on the sealing jutsu while the the other side might patrol the surrounding area. When an agent to Akatsuki, or possibly even a member, dies, Zetsu is sent to devour their bodies as to not have their secrets revealed. Due to the role he plays, Zetsu is the only member of Akatsuki to act without a partner on a regular basis. Despite this, Zetsu serves as a sort of superior to other members, as they often report directly to him instead of the actual leader, Pain. Zetsu is also the one sent to retrieve the rings of dead Akatsuki members.

Zetsu has been shown having his complete loyalty to Akatsuki, to the extent that the leader, Pain, trusts Zetsu with the knowledge of his identity. As a possible reward for his loyalty, Zetsu was once given a subordinate, Tobi, who would later become a member of Akatsuki himself. Zetsu is also shown knowing that Madara Uchiha is the founder of Akatsuki. He knows Tobi is Madara, since in chapter 395 he reports to Madara who at the time is wearing the mask that is identified with Tobi. During a conversation with Madara, Zetsu mentions that "It's been so long..." implying that he has known Madara and been part of his plans for some time.

Abilities
Little is currently known about Zetsu's abilities, though it has been seen he uses whatever abilities he has to help his hunter-nin role within Akatsuki. The only technique he has been shown using is his Mayfly technique, with which he merges with the ground to instantly travel to a new location. It has also been noted that Zetsu has the ability to "record" the things he sees. After the battle with Sasuke and Itachi, Madara Uchiha asks him "Did you make sure to record the whole thing while you watched?", and Zetsu replies, "Relax, I got everything". He may also be able to allow anyone to view what he's seen, noted when Madara says he'll "enjoy the viewing later", or it could just be Madara, with the use of his Sharingan, reading Zetsu's mind. He has also shown the abillity to split in two, when the white side leads Taka to the Kage meeting while the black half stays with Madara.

Sasuke Retrieval arc
Zetsu's first appearence in the series was just after Naruto Uzumaki's and Sasuke Uchiha's battle at the Valley of the End. Just as the two delivered their final attacks, Zetsu appeared in the distance, as if surveying their power. He later appeared with the other Akatsuki members during a meeting about their plans and the situations of the world around them, commenting on how Orochimaru now had access to the Sharingan via Sasuke.

Rescue Gaara arc
Zetsu makes a Part II appearance while Akatsuki seals the One-Tailed Shukaku to tell Pain that a rescue attempt for Gaara, whom the Akatsuki had captured, is already in motion. Following the defeats of Yura and Mukade from Sunagakure, Zetsu travels to their dead bodies' locations to devour them and destroy any secrets their bodies left behind. Following Sasori's death, Zetsu and Tobi travel to their former headquarters to retrieve Sasori's and Deidara's rings, where Zetsu sees Sasori's true form for the first time. While Deidara still lives, Tobi finds Sasori's ring, and believes he could be a member with it - Zetsu remarks that it's not that simple (Zetsu appears to be hiding Tobi's true identity to Deidara by mentioning this).

Hidan and Kakuzu arc
Zetsu's only notable appearance in this arc is to retrieve the body of Yugito Nii, the jinchūriki Hidan and Kakuzu successfully captured, and take it to their new headquarters; This has allowed the two to track other targets, particularly Naruto Uzumaki. However, Zetsu is also shown appearing in all Akatsuki meetings during this arc.

Hunt for Uchiha arc
Zetsu, following the death of Deidara, tells the other members that both Deidara and Tobi have died alongside Sasuke Uchiha (though only Deidara died in reality). Zetsu is also shown having watched Pain's battle with Jiraiya, and following the battle Zetsu's two halves argue over whether to watch Pain's battle with Jiraiya or Itachi Uchiha's battle with Sasuke Uchiha. He chooses Itachi's fight. As they watch the fight, Zetsu's white half wonders how Sasuke beat Itachi using only the normal Sharingan, while his black half claims that the Sharingan is just a weapon, and the skill of a weapon depends on its user. After Itachi dies and Sasuke passes out, Zetsu leaves the area, and reports to Tobi. After a short battle with the 8 Man Squad containing Naruto, Sakura, Kakashi, Sai, Yamato, Kiba, Shino, and Hinata, the two returned to the battle area where Sasuke and Itachi's battle was although Zetsu did consider about having revenge against Naruto for being called 'aloe'. They retrieved the two bodies before the 8 Man Squad did and took their leave. Zetsu is last seen at an unknown Akatsuki hideout.

Sasuke & The Eight-Tailed Beast arc
Zetsu is seen with Madara and Kisame sealing the Eight-Tails (which Sasuke successfully captured) in another hidden Akatsuki hideout. In a surprised twist of events, the host explodes into smoke, reverting back into a tentacle of the Eight-Tailed beast. Madara is shown to remain silent, as Zetsu's white half bursts out laughing, which his black half scolds him for. Kisame merely states that Sasuke had been fooled by the Jinchuriki, with a smirk.

Invasion of Pain arc
After Naruto's fight with Nagato, Zetsu appears on a far-off tree, watching Naruto being cheered by all of Konoha's inhabitants. Zetsu's white half says he never thought that Pain would be killed. The black half then says that he should report this news and he goes to tell Madara. After reporting, Zetsu also mentions that Konan will not be returning. Madara says that they'll need to find a replacement for her to synchronize with the Demonic Statue of the Outer Path. He then orders Kisame to hunt the Eight-Tails while he tends to other matters.

The Summit of the Five Kage arc
While Taka is heading to the Konohagakure, Tobi appears and tells them if they betrayed the Akatsuki they would die. Shortly after Tobi's arrival, Zetsu appears stating that Danzo is the new Hokage. Zetsu then splits apart and guides his Taka with his white half and his black half stayed behind with Madara.

Creation
In Naruto Hiden: Sha no Sho Character Official Data Book, Kishimoto reveals that he'd originally planned for Akatsuki to be a group of monsters, "with almost no human characteristics," and that Zetsu's design was modeled after an insectivorous plant. He also states that he decided to make Zetsu half black and half white to, "better show his split personality."

Trivia

 * In fan videos and drawings, it is popular to call the Black half Zet and the white half Su.
 * Because of his plant-like appearance, many fans believe Zetsu to be formerly of Kusagakure. However, in the 3rd data book, the spot of his village of origin in his profile is left blank. His lack of a Grass headband with a slash mark across it also contradicts the Kusa theory.
 * According to Pain, Zetsu's reason for fighting is "Land".
 * Out of all the Akatsuki's members, Zetsu is the only one who hasn't been seen fighting in a battle yet.
 * Zetsu was the third Akatsuki member to be shown in the series that was was still an active member at the time.
 * Zetsu is a cannibal, he usually eats corpses of dead associates of the Akatsuki, sometimes dead members of the Akatsuki, and/or dead hosts of the Tailed Beasts.
 * According to the Third Databook, wherever Madara is, Zetsu isn't far behind.
 * Zetsu's card in the Naruto Shippuden card game shows that he is from Konoha although this could be just a symbol to fill the space where the village goes or is a mistake.

Quotes

 * (To Tobi about joining Akatsuki) Black side: "Idiot. It's not that simple." White side: "It'll be all right. Tobi is a good boy."
 * (To Hidan) White side: "When you're sad and alone..." Black side: "...all you can count on is yourself."
 * (When Kirabi's body turns into a tentacle during extraction) White side: "Ahahaha a tentacle!" Black Side: "Don't laugh!"