Orochimaru

Orochimaru (大蛇丸, Orochimaru) served as the central antagonist for the majority of the series during Part I but later became a secondary antagonist during Part II. Recognised as one of the most powerful ninja Konohagakure ever produced and one of the "Three Legendary Shinobi" (伝説の三忍, Densetsu no Sannin), he operated as an ANBU shinobi within the Root faction prior to his defection from the village in pursuit of his own self-serving ambitions. Initially sealed away during the battle between Sasuke and Itachi Uchiha by the latter, he was later revived by the former using his own genetic material that was within his underling Kabuto Yakushi and the portion of consciousness he stored within Anko Mitarashi's Cursed Seal of Heaven. 

Source: http://naruto.wikia.com/wiki/Orochimaru

Background

Orochimaru as a child.

Orochimaru was an orphan who became a pupil of Hiruzen Sarutobi alongside Jiraiya and Tsunade. Compared to the more laid back Jiraiya, Orochimaru stood out as a genius — his talents, knowledge, and determination were considered by Hiruzen to be that of a prodigy seen once in a generation. According to Tsunade, Orochimaru had a twisted personality even as a child. His sadistic attitude was presumably due to the death of his parents. At some point after losing them, Orochimaru found a white snake near his parents' grave, with Hiruzen explaining that it represents fortune and rebirth inspiring Orochimaru to study kinjutsu and obtain knowledge of all techniques. Jiraiya theorised that Orochimaru went down this path in an attempt to forget his painful memories.
During the Second Shinobi World War, Orochimaru joined Jiraiya and Tsunade in fighting Hanzō, who gave them the title of Konoha's "Three Legendary Shinobi" (伝説の三忍, Densetsu no Sannin) as a reward for surviving their battle with him, while the rest of their group were easily slaughtered in the process. On their way back to Konoha, they encountered three Amegakure orphans, with Orochimaru proposing a mercy killing before Jiraiya chose to train them instead. In the anime, Orochimaru orchestrated the last known rampage of the Eight-Tails in Kumogakure by infiltrating the village while disguised as a Kumo-nin and posing as Fukai's doctor in order to trick him into taking special genjutsu inducing pills to bring forth the Eight-Tails to attack Kumogakure. After the rampage, Orochimaru obtained that severed horn of the Eight-Tails's horn and harvested Fukai's DNA from it.

Orochimaru and his student, Anko.

Some time after Team Hiruzen was disbanded, Orochimaru became an ANBU member and joined Root to work directly under Danzō Shimura, while also becoming a mentor to Anko Mitarashi. It was during that time that Orochimaru met Kabuto Yakushi while accompanying Danzō in forcing the boy's guardian Nonō into a mission. While Orochimaru's ambitions did include becoming the Fourth Hokage, he intended to use the title for his own agenda.Though Hiruzen knew the potential evil his former pupil had in him, he hoped to stir Orochimaru from the path before slowly realising that he was beyond anyone's help.

Orochimaru encounters Kabuto again after he retreated from Iwagakure.

Orochimaru was later sent on a mission by Danzō to Iwagakure to ensure that both Kabuto and Nonō, the former having joined Root as a spy, killed each other as they had become major risks. Orochimaru, instead, spirited Kabuto to his hideout, what would become part of his personal hidden village, Otogakure, and revealed Root's intent for Kabuto's life and the role he was to play in it. Though Orochimaru revealed that he was also sent to kill the one who survived this ordeal, he told Kabuto that he saw a bit of himself in the boy and decided killing him would be a waste of great talent. Therefore, Orochimaru offered Kabuto a position as his right hand and spy with the promise of an identity.
With Kabuto's assistance, he performed various experiments on his prisoners, some being fellow Konoha shinobi he kidnapped. He uses them as human guinea pigs to develop techniques that would grant him immortality, the end result being Living Corpse Reincarnation. Some time later, Orochimaru was at the outskirts of Kirigakure as he found Kimimaro, the sole survivor of the Kaguya Clan. Soon after, thanks in part to Kimimaro, Orochimaru's research on the source of Jūgo's powers led him to find the Ryūchi Cave and master senjutsu.^[13] Unable to enter Sage Mode because of the fragility of his host body, Orochimaru instead developed an alternate method of harnessing his senjutsu chakra through cursed seals, which he tested on Anko and several others before eventually using the finalised versions on Kimimaro and the Sound Four.
When Hiruzen finally retired and named Minato Namikaze his successor, Orochimaru saw no more reason to remain in Konoha and began to be less discreet with his actions. This led to his experimentation on the cultivation of Hashirama Senju's DNA with sixty children he kidnapped to recreate the First's Wood Release techniques, having already done so with Danzō, while also endowing the elder with multiple Sharingan. However, as his test subjects began to die off, Orochimaru was caught red-handed by Hiruzen, who knew it was his duty to kill him yet lacked the will to do so. Leaving Konoha and his emotionally broken mentor, with Jiraiya's pleas for him to reconsider falling on deaf ears, Orochimaru was unaware that one child had survived his experiment.

Orochimaru attacks Itachi.

Orochimaru soon joined the elite criminal organisation Akatsuki, his role in which was unclear, although he was partnered with Sasori, whom he did much good for the Akatsuki with. However, when Itachi Uchiha joined Akatsuki after conducting the Uchiha clan massacre, Orochimaru saw his opportunity to steal his body to gain possession of the Sharingan. Itachi used his Sharingan to stop him from doing so, while also cutting Orochimaru's left hand off to prevent him from breaking the genjutsu he was under, which lead to him leaving Akatsuki to acquire a new body. For Orochimaru's defection, Sasori developed a grudge against him and used Kabuto, whom Orochimaru had sent to serve as his spy within Akatsuki, to counter spy as his sleeper agent. However, Orochimaru discovered and undid the technique Sasori had used on him.Orochimaru had also been on a life-long search for the Totsuka Sword, an ethereal sword, but was unaware that it was in the possession of Itachi's Susanoo.

Personality

Orochimaru possesses a complex personality, but more often than not, however, he was often described as twisted by a large number of people, including Tsunade. Jiraiya speculated that this was due to the fact that both of his parents died when he was at a very young age, along with his horror for Tsunade losing Nawaki and later shedding tears when she lost Dan, imply that Orochimaru grew to disdain the fragility of human life and how it affected those still living, which led him to desire immortality. To that end, the primary purpose of Orochimaru's human experimentation is to test what modifications the human body can endure and harness a subject's unique abilities for himself. Once accomplishing this goal, Orochimaru had hoped to be worthy of the title of "ultimate being", unable to die and able to achieve his secondary goal to learn every ninja technique in the world, which would normally take many lifetimes to obtain.
According to Sasuke, becoming cruel with any good he had as a member of Team Hiruzen gone, Orochimaru's agenda made him develop a god complex and valued himself while delighting in striking terror in his first impressions. Though causing conflict to ease his boredom, Orochimaru seems to have no interest in wars started by others. His charisma also attracts a number of followers, many of which he would not hesitate to send on potential suicide missions if for his own personal benefit, expressing some remorse if they're unable to properly complete a task, having no sentimental attachments to any of them despite their values. Despite this, Orochimaru has genuine interest in the development of his underlings and experiments.
Taking Sasuke as an apprentice, seeing him as a prodigy like himself, Orochimaru intended to take the youth's body to obtain the Sharingan to reach another step in his goal of immortality and learning every technique in the world. This goal has developed to the point of obsession. However, despite knowing that Sasuke was willing to do anything for revenge, including sacrificing his own body, Orochimaru did anticipate treachery from his disciple and showed only a small level of surprise when it actually occurred. What truly surprised Orochimaru was Sasuke's decision to follow his own path and ultimately choosing defend the village he initially swore to destroy, one that Orochimaru openly approved and curious to see what the end result it.
He also had an initial disdain toward Naruto Uzumaki, believing him to be a poor excuse for a ninja until he defeated Kabuto with the Rasengan. Despite attempting to kill Naruto at the time after coming to the realisation that he would become a threat to him in the future, Orochimaru became interested in Naruto's development, provoking Naruto during their battle to see his full abilities at the time. Though coming to hate his home village, orchestrating a few attempts to destroy it, Orochimaru despised his former Akatsuki colleagues more. Throughout Part II, if Orochimaru was not busy with Sasuke's development, he would try to cripple the Akatsuki's operations, seen in the "Sai and Sasuke Arc" as well as the "Three-Tails Arc". He expresses delight over the death of Sasori and later convinced Sasuke from killing Team Kakashi so they would thin out the Akatsuki membership.
An aspect of Orochimaru's personality is also his greatest weakness: his arrogance. He truly believed he is an invincible immortal at times, attacking recklessly against powerful opponents such as his former teacher and Itachi Uchiha. Both times he was left crippled in some way since he either wanted to make his opponent suffer, like against Hiruzen, or because he was too uninformed to face his opponent, like against Itachi. Despite his distinct lack of morals, Orochimaru seems to be well-versed in human psychology and displays insightful wisdom when disparaging Tobirama for the actions resulting from his policies.
After being revived by Sasuke, his personality changed a bit. While he still wished that he could gain Sasuke's body, he acknowledged that he was too weak at the time. He also realised that copying someone completely was a mistake from his observation of Kabuto and displayed interest in Sasuke's "different path". Later, he felt sentimental over his failed candidacy for being Hokage, though this may be because he was in front of his master and the Fourth. Wanting to see Sasuke's path, he chose to fight alongside the Uchiha instead of ignoring Madara's war like he planned, and even made it a priority to heal Tsunade and the other Kage after their defeat before hurrying to the battlefield. Orochimaru also shows a sick sense of humour at the time, from using the memory of Sasuke stabbing him to convince Karin to aid the youth again to his comical method of threatening her and Suigetsu into cooperating to heal Tsunade.

Appearance

Orochimaru.

Originally, Orochimaru was a tall man with extremely pale skin and waist-length black hair. He had a long face with pronounced cheekbones, amber eyes with slits in his pupils and purple markings around his eyes  a reference to his snake-like nature. Although he has taken on new bodies repeatedly, he has modified them each to resemble his original form, minus the height differences. He apparently used his Vanishing Facial Copy Technique on his original body so he can wear it over his hosts' faces as a mask.

Orochimaru's summoning tattoo.

Orochimaru is sometimes seen sporting different attires. He usually wears plain grey garbs with black pants and a thick purple rope belt tied in a large knot behind his back and blue tomoe-shaped earrings. He is seen wearing a black polo neck under it. As a young child, he wore long white robes with purple lining embroidered with black or yellow tomoe markings, tied with a purple ribbon forming a bow in the front, as well as white leggings and black open-toed shoes, and mesh top with a purple lining under it. In various flashbacks, he is also seen wearing the Oto ninja uniform and forehead protector, as well as the Konoha ninja uniform. During his time in Konoha, his casual wear usually consisted of a dark kimono shirt with violet lining over a mesh shirt, bandages that he uses as a sash, dark pants, and armour around his hips. On his left hand, he has an intricate tattoo that he uses to summon snakes. On one occasion, in which he proposed the request for Tsunade to heal his arms, he was seen wearing long purple robes with a loose pink vest, as well as a off-grey kimono with a polo neck in Zaku's flashback.

Orochimaru's Akatsuki attire.

During his time with the Akatsuki, he wore the standard red and black Akatsuki robes. Orochimaru wore his slate blue Akatsuki ring, which bears the kanji for "sky" or "void" (空, kū), on his left little finger. In his room, he is sometimes seen wearing a mahogany shirt with a design of two white snakes on the back of the sleeves to the back on both sides, as well as black pants.
After countless experiments with his body, his true form became a giant snake ("The Great White Serpent") made up of a multitude of smaller snakes, with long and spiky black hair, a scaled face with snake-like teeth and a long tongue, black eye markings, and a pointed chin.
After being resurrected by Sasuke, Orochimaru donned a black cloak and hood somewhere between his rebirth and arriving to Konoha.

Abilities

Being one of the legendary Sannin, as well as a former member of the S-rank criminal organisation Akatsuki, Orochimaru is an extraordinarily powerful ninja, being Kage-level in his own right. He was considered as a child prodigy that appeared only once a generation. His skills were so great that during the time of his invasion of Konoha, the Third Hokage feared no one in the village would be able to defeat him or even fight on equal footing with him, even himself, and that Orochimaru was strong enough to take down an entire small nation on his own. Even Kakashi Hatake, one of Konoha's greatest shinobi, was filled with tremendous fear when he met him during the Chūnin Exams, and himself admitted that he wouldn't even stand a chance against his level of power. Sasuke Uchiha and Suigetsu Hōzuki also stated that the former's victory over Orochimaru was only because the latter was already severely weakened prior to their confrontation.
During his battle against the Third Hokage, his arms were sealed by the latter, preventing Orochimaru from forming hand signs and effectively stripping him of most of his technique arsenal. Despite this, he was able to hold his own against a four-tailed Naruto, plus while his body was greatly weakened. Even with the danger Naruto imposed, Orochimaru found the battle highly entertaining, even playing with Naruto's initial three forms. Even when his body gave in, he expressed regret that he couldn't complete the battle. Even among the Sannin, he was the one who originally stood out for individual ability. Due to him rarely being shown fighting, he has only displayed a limited amount of techniques, though his fighting style is very fluid and it stands to reason that in his efforts to learn all existing techniques, his array of abilities has probably become larger than any other single person and that he has vast knowledge on all forms of techniques. After acquiring the Uzumaki's mask of the Shinigami, he was able to break the seal on his arms, regaining his former strength.

Ninjutsu

Orochimaru shedding his body.

Wishing to obtain all of the techniques in the world, Orochimaru has a vast arsenal of techniques at his disposal, ranging from simple offensive ones, which nonetheless become deadly when performed by him, to higher ranking, more complicated ones. Orochimaru is also very adept with transformation, as he was able to view Sasuke's battle with Yoroi Akadō during the Chūnin Exams without any suspicion. He was also able to pose as the Fourth Kazekage for weeks, fooling the entire village of Sunagakure, including its elite shinobi and the ones closest to the dead Kazekage.
He is also able to use Wind Release: Great Breakthrough, through which he can level a wide area with a powerful gust of wind. In terms of defensive techniques, Orochimaru uses both Summoning: Rashōmon and Summoning: Triple Rashōmon, in which he summons a varying number of large demonic gates that are capable of enduring even the most intense forms of damage. Orochimaru can also use a technique to avoid damage, and another to temporarily paralyse an opponent. In the anime, of the few techniques he was seen using is the Earth Release Shadow Clone, which creates a copy of himself out of mud that can attack on his behalf.