Zetsu

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

Personality
Zetsu is easily distinguished by two large venus fly trap-like extensions enveloping 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 with their thoughts. 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 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 have been in the works for a long time.

Rescue Gaara Arc
Zetsu appears frequently throughout the series. 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 report directly to him instead of the actual leader, Pain. In addition to serving his various Akatsuki duties, he watches battles of interest and then reports the outcome to the other members.

Hunt for Uchiha Arc
During the fight between Sasuke and Itachi, he also acts as a commentator, giving the reader some insight into the techniques both of the Uchiha are performing. He later appears to inform Tobi of the result of the fight and then leads him there, where Madara asks if he has recorded everything, something Zetsu confirms.

Summit of the Five Kages Arc
After Itachi's defeat and Sasuke's recruitment, Zetsu takes a more active role in Madara's ongoing plans by marveling at the effort that went into getting Sasuke to work with Akatsuki. He is later revealed to have watched the entire fight between Pain and Naruto Uzumaki and appears next to Tobi as he argues to Sasuke about what his next objective is. There he reveals that Konoha has been destroyed and when Tobi asks he reveals that Danzo has been named the Sixth Hokage. He then proceeds to explain the outcome of the battle between Pain and Naruto. When Taka agrees to go to the Kage Summit, Zetsu reveals his ability to split in half and the white half leads Sasuke to the Kage summit.

The black one stays behind and discuss with Madara that they might lose control over Sasuke and would lose their last chance to have someone sync with Demonic Statue of the Outer Path. Madara then declares that he is tired of hiding and explains that they need to speed up the "Moon's Eye Plan".

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 asks him "Did you make sure to record the whole thing while you watched?", and Zetsu replies, "Relax, I got everything". It is unknown how he is showing these "Recordings".

He has also shown the ability to split in two, allowing his two personalities to act independently of one another.

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".
 * 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!"