Black Zetsu

An agent of the Akatsuki who functions as the organization's spy and is Tobi's Right Hand. Zetsu initially poses as a servant to the Akatsuki, Tobi and Madara Uchiha, before revealing his true allegiance to Kaguya Otsutsuki.

Background
As Black Zetsu later revealed, he is actually an extension of Kaguya Otsutsuki's will, while the entire White Zetsu species is what humans gradually turn into as they are bound to the God-Tree. Black Zetsu was originally conceived from Kaguya's thoughts before she was assimilated by the Ten-Tailed Beast and sealed into the moon. Black Zetsu spied on the Sage of Six Paths, before moving onto his son Indora after noticing his aggressive nature. Black Zetsu persuaded Indora to wage war against his brother Ashura, a feud that would span over a thousand years. Black Zetsu patiently watched over Indora's descendants, now known as the Uchiha clan, in the hopes that one of them would awaken the Rinnegan, which would provide the means for reviving Kaguya. When Madara Uchiha was fatally wounded in his battle with Hashirama Senju, Black Zetsu revived him while hiding in his body, allowing Madara to plot their revenge.

White Zetsu was originally one of the many victims of the Ten-Tailed Beast. He was hypnotized and absorbed into the beast's tree-form until he eventually became a plant-human hybrid. Madara Uchiha used the DNA of Hashirama Senju and the power of Demonic Statue of the Outer Path to summon the White Zetsu from the statue to act as his servants. Unlike his developing siblings, Zetsu was only semi-formed and yet his intelligence gathering was vital. In time, he and another deformed White Zetsu became friends with Obito Uchiha as he was rehabilitating.

Eventually, when Madara was near death after teaching Obito everything he knew, the man grafted Black Zetsu onto White Zetsu to aid Obito in completeling his life-long plan. Now a complete being, with two personalities, Zetsu became Obito's right hand while playing the role of his master's subordinate when he first dons the Tobi alias. Years later, Zetsu witnessed at the battle between Naruto Uzumaki and Sasuke Uchiha at the Valley of the End, reporting this event to the Akatsuki.

Plot
Zetsu appears frequently, usually to serve his various Akatsuki duties, observe battles of interest to report the outcomes, and devour the corpses of dead associates and Akatsuki members. After Sasori's death, to eat his corpse and the presumed dead Deidara's, Zetsu is accompanied by Tobi to find the deceased member's ring. Though Zetsu is displeased to find Sasori being a puppet and though inedible, he was a bit conflicted when Tobi decides to become an official member to fill the vacant spot Sasori life. Zetsu also witnessed Sasuke's fight with Itachi Uchiha, noticing the change in the latter's fighting style before leaving to tell Tobi of fight's result. Though White Zetsu took offense to Naruto calling him "aloe vera", Blck Zetsu tells him to ignore it at they lead Tobi to Sasuke.

After Sasuke's recruitment, and the number of pawns within the Akatsuki dwindling, Zetsu took a more active role in Tobi. The first being to for White Zetsu to guide Sasuke and his group Taka to the kage Summit to assassinate Danzo Shimura, with Black Zetsu remaining by Tobi's side to provide him council over the recent chain of events while update him on Taka's progress. However, unknown to Taka, Zetsu's mission was to expose them to the Kage to cause a situation that Tobi hoped would resulted with the Kage defeated so he would hold them ransom to get Naruto and Killer Bee. When the plan failed, helping Sasuke during his fight with Naruto, Tobi orders White Zetsu to fall back and rejoin Black Zetsu to aid Kisame Hoshigaki in his plan to infiltrate Kumogakure.

As the Fourth Great Ninja war commences, with his clone brothers all deployed to fight the Allied Shinobi Forces, Zetsu splits into two again with White Zetsu remaining by Sasuke's side to oversee his progress and protect him from Kabuto Yakushi while Black Zetsu is given the mission to hunt down the five Feudal Lords and hold them hostage. But once Naruto and Killer Bee arrive to the battle to make his mission moot, Black Zetsu is given new orders to hold the Fudeal Lord protection group, led by the Fourth Mizukage Mei Terumi, at bay and keep them from regrouping with the rest of their forces. By the second day of his guerilla warfare, as his White Zetsu counterpart was destroyed by Sasuke testing his Eternal Mangekyo Sharingan on him, Black Zetsu finds himself fighting one of Naruto's shadow cloned before being pinned down Chojuro.

While the other ninja are fighting Obito, Madara and the Ten-Tailed Beast, Black Zetsu silently sneaks onto the battlefield. As Obito is defeated, Madara controls Black Zetsu and has him fuse to Obito, whom he then forces to use the resurrection technique that would revive him. Stating that he'll retrieve Madara's Rinnegan, Black Zetsu uses Obito's dying body to fight off Kakashi and Minato. As minato tries to seal his portion of the Nine-Tailed Fox into Naruto's dying body, Balck Zetsu temporarily loosens his hold on Obito to step in and take the Fox for himself.

When Madara orders Zetsu to hand him Obito's Rinnegan, Obito resists and steals some of Madara's power before teleporting away. Madara steals Kakashi's Sharingan and follows Obito. When they teleport back, Madara has taken Obito's Rinnegan while Black Zetsu has both Sharingan and Obito's now dead body. After Madara finally casts hin illusion across the earth, Zetsu declares his task done and stabs Madara in the back. He then reveals that he only pretended to be Madara's servant when the true source of his life came from someone else, Madara's ancestor Kaguya Otsutsuki. Zetsu then assimilates Madara the same way he assimilated Obito. After Kaguya corners Naruto and Sasuke, Black Zetsu extends from her to siphon their chakra, all the while telling them Kaguya's history. Naruto is able to shake him off.

Rock Lee & his Ninja Pals
Zetsu made a minor appearance in the chibi spin-off Rock Lee and his Ninja Pals. As opposed to the original series, Zetsu is portrayed as a single person instead of having split personalities, notably having Black Zetsu's voice. He made a minor appearance in the credits where he joins his teammates fight Konoha 11. Zetsu then made a minor appearance in Episode 30 in Tobi's story under the name of Zetsy and in Deidara's story as a referee of the race & then in person near the end of Episode 38. He arrived with Tobi, who is possibly his current partner, on their hideout & then wondered what happened there upon seeing it destroyed. Sasori upon seeing him starts to blame him for "recruiting" Lee & Guy into the Akatsuki. Zetsu rejects what Sasori said but Hidan proves what the latter said is true by describing Lee's physical appearance in which Tobi easily recognised. Upon learning the truth from Tobi, everyone in Akatsuki (and possibly Zetsu) beat up Deidara for not telling Lee was a spy.

Appearance
Zetsu is easily distinguished by two large venus flytrap-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 short green hair, yellow eyes, and that his body has two different colored halves: The left side is completely white with a resemblance to Hashirama Senju and the right side mostly black, the hand and feet white, with almost no facial features. When he splits into two the plant like extensions go down the waist and now have the appearance of spikes, each halve has half of the Akatsuki robe and the halve they are missing is replaced with new body mass.

Unlike the white side, which shows a normal mouth, nose, and eye, the black side shows no visible orifices at all and only shows the iris of the eye. The black side also has white dots along the edges; these are not flat markings, but rather protrude from his body, like little white bumps. Zetsu also has unusually rounded teeth. He has a red nail polish for his fingers (his toenails, seen at his debut, are bluer) and wears his ring on the right little finger. In the manga, Zetsu does not have fingernails.

Personality
According with his two differently colored halves, Zetsu seems to have a split personality, as in the Japanese version of the manga, the black half of his face only uses katakana for okurigana and furigana while the white side uses kanji and kana regularly, 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 knowledgeable while the white half is carefree, easygoing, and seems to enjoy toying with others). They also seem to communicate through their thoughts.

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. The character showed some lust for money when he quoted Kakuzu on his principle that money is most important. However, whether or not this applies for both personalities is as of yet a mystery to be dealt with. Zetsu's white side has also shown to be very confident in himself as he was willing to fight Naruto despite knowing Pain had lost in battle against him. White Zetsu was one of the few characters that had a positive relationship with Obito, as they showed no signs of betrayal or resentment. Black Zetsu, however, was never truly loyal to anyone except Kaguya Otsutsuki, stating that his only purpose in life is to help Kaguya's schemes come to fruition. He even ridiculed both Obito and Madara - the two men who had done the most damage in the series - as useless, stating that he himself was a more effective follower.

Zetsu's main role within the Akatsuki is to function as a spy, and often performs duties that are similar to a hunter-nin. 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, which adds further his venus fly trap theme. 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 Figurehead leader, Pain. He in turn would report to pain as well as the Actual leader Tobi.

Abilities
Though normally a spy, Zetsu's powers make him not a 'front line' fighter. Instead, Zetsu uses the jutsu Mayfly where he merges into the ground to instantly travel to a new location, though he isn't nearly as fast as Tobi's Kamui but ideal for spying along with a photographic memory.

Zetsu has also shown the ability to split in two, so that his two personalities can act independently. He is also able to use a time-delayed spore jutsu to grow into a mushy mass around several individuals at once, thereby sucking their chakra out, and can even use it to replenish another persons' chakra. Zetsu's white half has also shown the ability to create multiple copies of himself. Zetsu can create a special clone which can simulate the appearance anyone Zetsu has ever touched down to their chakra, but it is much weaker than the original person, and therefore mostly only useful as a diversion.

All Zetsus have the ability to either steal chakra from other ninja or encase them like a suit.

White Zetsu Army
Prior to the Fourth Great Ninja War, Tobi used the chakra of the seven Tailed Beasts he obtained to modify White Zetsu's 100 thousand clone brothers into more human-like forms. With Yamato's Wood Style added into them, the White Zetsus become more battle oriented as they engage the Allied Shinobi Forces. However, Naruto's involvement greatly diminished the White Zetsu army during the first night.

Trivia

 * White Zetsu's line "It'll be all right, after all, Tobi is a good boy" to Black Zetsu upon convincing him to let Tobi "join" Akatsuki became a popular meme for Tobi leading to the birth of "Tobi is a Good Boy".
 * Black Zetsu is a villainous counterpart to the Sage of the Six Paths:
 * He considers himself the son of Kaguya, but was conceived asexually
 * He has black skin while the Sage has white
 * He is obedient to his mother while the Sage was rebellious
 * He steals power while the Sage shares it
 * Black Zetsu reveals that he manipulated Indra Otsutsuki to turn against Ashura, rewrote the text on the Uchiha monument into tricking the Uchiha into thinking that the Moon's Eye Plan will bring peace to the world, thought up the idea to forming Akatsuki, and showed Kabuto where Madara's corpse was thus making him the real main antagonist of the series.