Black Zetsu

Black Zetsu is an agent of the Akatsuki who functions as the organization's spy and is Madara's right hand man.

He initially posed as a servant to the Akatsuki, Tobi and Madara Uchiha, before unveiling his true allegiance to Kaguya Otsutsuki, whose's will he was created from. Due to the events that were caused by him since the beginning of the series's timeline, Black Zetsu is the true main antagonist of the Naruto series.

Background
As Black Zetsu later revealed, he is actually a nameless 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 final will 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 still-living remains of 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. At that moment, Madara believed that White Zetsus was his creations that made from both power of Demonic Statue and Hashirama's DNA, while in reality, Black Zetsu was released them without he knowing (as Madara uses these unleashed Zetsusas material to create his own Zetsus with Hashirama's DNA). 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 completing 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", Black 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 his illusion across the earth, trapping everyone in the Infinite Tsukuyomi before also performing the God: Nativity of a World of Trees which eventually wraps those caught in the dream into trees but thanks to Sasuke, Team 7 of Sasuke, Naruto, Sakura and Kakashi are spared.

Upon seeing them, Madara arrogantly dubs himself the Savior of the World, one who has saved the world from itself by replacing the hells of reality with the heavens of dreams but as he rambles, Black Zetsu appears, stabbing Madara in the back, shocking everyone.

As Madara, distraught struggles to adjust to this betrayal, he states that Black Zetsu was his will but Black Zetsu states that it has never served Madara and that he is the will of Madara's ancestor Kaguya Otsutsuki.

Zetsu then assimilates Madara the same way he assimilated Obito with the move bringing Kaguya into the modern world.

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 and separates Zetsu from the battle by chopping off Kagya's left arm.

After Kaguya is defeated, Black Zetsu decides to retreat for now, vowing to wait for another opportunity to revive Kaguya and begin again.

Unfortunately for Black Zetu, his dreams are shattered when Naruto sends him into Kaguya's prison, trapping the two there for eternity with no possibility of them ever returning to inflict any harm on the ninja world, while at the same time ironically fulfilling Black Zetsu's desire to never leave his mother's side.

By sealing Black Zetsu alongside Kaguya, Naruto has defeated the two biggest threats the world has ever known. With no-one around to carry out Kaguya's will, the shinobi world will now be able to see an end to their conflict and finally bring about a new age of peace and understanding.

Rock Lee & His Ninja Pals
Zetsu made a minor appearance in the chibi spin-off Rock Lee & 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 recognized. 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. 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.

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 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. The dots disappear shortly before Zetsu splits. Black Zetsu is actually a whole being, but keeps his other half hidden. He's also capable of turning into liquid and forming a wide empty grin with a stitch-pattern echoing sharp teeth.

Personality
Compared to White Zetsu, Black Zetsu, nicknamed "Wicked Tongue" (毒舌, Dokuzetsu), is more serious and knowledgeable. In the Japanese version of the manga, Black Zetsu only uses katakana for okurigana and furigana. This is often conveyed in the anime through a deeper voice. While it and White Zetsu sometimes had differing opinions, the two halves were still able to function as a single entity.

Black Zetsu had the skill of appearing extremely trustworthy, obedient and loyal, being entrusted with Akatsuki's secrets and knowledge of Madara's Eye of the Moon Plan. In actuality, however, its true loyalties were kept secret from nearly everyone, including Madara himself. Unlike White Zetsu, who was kind to his allies, Black Zetsu expresses its belief of anyone beneath Madara as useless. When Obito lost the tailed beasts, Black Zetsu commented on how it was a more effective servant to Madara. It also had no qualms about using Obito's dying body as a host when it fought Kakashi Hatake and Minato Namikaze, showing how little it cared about the Uchiha's welfare, while deriding Obito for being useless for his treason. However, Black Zetsu's loyalties do not truly extend to Madara either, having feigned being created by the man in order to manipulate him discreetly, and then harshly betrayed him and expressed to him how he was really no different from Obito. Black Zetsu even refers to Hagoromo's ideals of ninshū as absurd and his original text on the stone tablet as garbage.

Zetsu's main role within Akatsuki was to function as a spy, with other duties that were similar to those of a hunter-nin. It was able to control its body and take part in the tailed beast sealing ritual simultaneously, which, in addition to being unique to it, allowed it to act as a lookout while the sealing took place. When an agent of Akatsuki, or possibly even a member, died, Zetsu was sent to devour their bodies so as to not have their secrets revealed, further adding to its nature as a half-plant creature. Zetsu was also the one sent to retrieve the rings of dead Akatsuki members.

Due to the role it played, Zetsu was the only member of Akatsuki to act without a partner on a regular basis. It operated on its own, often moving between the different locations where the other Akatsuki members were located. This resulted in other members reporting directly to it, rather than the figurehead leader, Pain. Zetsu would in turn report directly to Pain as well as Obito, the organisation's benefactor and eventual leader.

Black Zetsu is generally harsh and critical of nearly everyone it comes across, with the exception of Kaguya whose will created it. Referring to itself as her son, it shows a deep reverence and devotion to her and is willing to go to any lengths to please her. It even goes as far as to encourage others to sympathize with her predicament following her sealing. Black Zetsu's determination to aid the one it calls mother runs so deep and is so overwhelmingly strong that it spent countless years influencing the shinobi world in order to facilitate her revival with Black Zetsu ever going so far as to arrogantly claim that the history of shinobi exists only to bring Kaguya back. Believing itself to be the progenitor of the shinobi history, Black Zetsu thinks very highly of itself, and grew angry at Naruto for even daring to touch it, believing him to be unworthy to do so.

Interestingly, when it first referred to itself as Kaguya's child, Black Zetsu's tone and demeanor was noted by Sasuke Uchiha to have drastically changed. It appears that in the end, Black Zetsu's true loyalties reach only to Kaguya Ōtsutsuki, a person it claims to be the will of, and whom it affectionately refers to as "mother" (母さん, kaa-san); Naruto even goes so far as to call Black Zetsu desperate to be at Kaguya's side, likening it to a snot-nosed brat who never wanted to leave its mother's side or rather, who is too afraid to leave its mother's side.

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 Zetsu incarnations 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 Ōtsutsuki 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.