Zetsu

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

Personality
Zetsu is easily distinguished by two large venus fly trap-like extensions that envelop his head and upper body, giving him a plant-like appearance. When his extensions are open and his head is visible, it is revealed that 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 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 through their thoughts. He also has unusual teeth. To further his venus fly trap theme, Zetsu is a cannibal.

Zetsu is seemingly very trustworthy, being entrusted with knowledge of Pain and Tobi's true identities. He has apparently been working with the latter for some time, Tobi first appearing as his "subordinate" and Zetsu later commenting that Tobi's plans had been in the works for a long time.

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. When an agent to Akatsuki, or possibly even a member, dies, Zetsu is sent to devour their bodies so as to not have their secrets revealed. Zetsu is also the one sent to retrieve the rings of dead Akatsuki members. 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 reported directly to him instead of to the actual leader, Pain.

Sasuke Retrieval
After Sasuke defeats Naruto at the Valley of the End, Kakashi and Pakkun appear to bring Naruto (and hopefully Sasuke) back home. As the three begin the journey back home to Konoha, Zetsu appears from the ground, having witnessed the whole battle.

Part II
Zetsu appears frequently throughout the series. In addition to serving his various Akatsuki duties, he watches battles of interest and then reports the outcome to the other members.

Rescue Gaara arc
Zetsu is seen spying on team Guy and team Kakashi and alerting Akatsuki. Later, he is sent to dispose the corpses of Mukade and Yura.

Hunt for Itachi arc
During the fight between Sasuke and Itachi, he also acted as a commentator, giving the reader some insight into the techniques both of the Uchiha were performing. He later appeared to inform Tobi of the result of the fight and then led him there, where Madara then asked if he had recorded everything, to which Zetsu confirmed.

Five Kage Summit arc
After Itachi's defeat and Sasuke's recruitment, Zetsu took a more active role in Madara's ongoing plans, marveling at the effort that went into getting Sasuke to work with Akatsuki. He was later revealed to have watched the entire fight between Pain and Naruto Uzumaki, and appeared next to Tobi as he argued with Sasuke about what his next objective was. There he revealed that Konoha had already been destroyed. When asked by Tobi, he revealed that Danzo had been named the acting Sixth Hokage. He then proceeded to explain the outcome of the battle between Pain and Naruto. When Taka agreed to go to the Kage Summit, Zetsu revealed his ability to split in half, and the white half proceeded to lead Sasuke to the Kage Summit.

The black half remained behind, and discussed with Madara the fact that they might lose control over Sasuke and would therefore lose their last chance to have someone sync with the Demonic Statue of the Outer Path. Madara declared that he was tired of hiding, and explained that they need to speed up the "Moon's Eye Plan".

The white half stayed with Taka, currently in the Land of Iron, observing the meeting of the Five Kages and trying to figure out a way to successfully assassinate Danzo. After identifying Danzo, the white half managed to leave Taka and slip into the conference room, telling the five Kages that Sasuke was somewhere around. At that time the black half was with Madara, both hidden in the Land of Iron and received the instruction to "begin".

After revealing himself, the Raikage grabbed the white Zetsu's neck, asking him the whereabouts of Sasuke, and threatened to use force. Zetsu mockingly replied that he would give a hint. The Raikage, angered, tightened his grip on Zetsu's neck. White Zetsu collapsed, leaving his ultimate fate unknown.

Abilities
Zetsu has yet to be seen in battle. 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 asked him, "Did you make sure to record the whole thing while you watched?" to which Zetsu replied, "Relax, I got everything." It is unknown how he is able to show these "recordings".

Zetsu has also shown the ability to split into two, so that his two personalities can act independently.

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

Trivia

 * Despite being the third member of Akatsuki to be introduced, he has not yet fought anyone.
 * Because of his plant-like appearance, many fans believe Zetsu to be formerly of Kusagakure. However, in the Third Databook, the spot of his village of origin in his profile was 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".
 * According to the Third Databook, wherever Madara is, Zetsu is never 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 possibly a mistake.
 * Zetsu is a complete and total badass.

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 turned into a tentacle during extraction) White side: "Ahahaha a tentacle!" Black side: "Don't laugh!"
 * Black Side: An expert with a stone can still beat a novice with a shuriken.