Kabuto Yakushi

is Orochimaru's former personal medic, assistant, and general right-hand man.

Background
As a young boy, Kabuto was found amongst the enemy dead after the Battle of Kikyo Pass by a chief medical officer of the Konoha Medical Unit Squad. The officer brought Kabuto with him back to Konoha, raised him as a son, and taught him medical jutsu when he got older. At some point in time, Kabuto was recruited by Sasori of Akatsuki, who sent Kabuto to spy on Orochimaru by using Brainwashing Investigation Technique to transform him into a sleeper agent. Orochimaru discovered this, and released Sasori's control over Kabuto's memories, leaving Kabuto with the option to follow him. With a little persuasion, Kabuto agreed to side with Orochimaru.

Kabuto has since used his experience with spying for Orochimaru, playing the role of Sasori's servant to gain knowledge of Akatsuki's plans. Through his affiliation with Konoha, Kabuto was also able to acquire the rank of Genin and enter the biannual Chunin Exams, using the first two parts of the exam to gather information on his fellow participants for Orochimaru's use. Additionally, with the medical expertise taught to him by his adoptive father, Kabuto is able to carry out human experiments for Orochimaru, while at the same time increasing his own knowledge of the human body.

Loyalty and Personality
Kabuto's loyalties and personality are difficult to discern, as he is a sadistic, yet polite, helpful man with a dry sense of humor. He enjoys playing mind games with his enemies, often unnerving them by pointing out their weaknesses and turning personal horrors against them. He seems to have a hard time tolerating rudeness though, indicated by his frequent admonishments of Sasuke Uchiha for not treating Orochimaru with respect. Orochimaru also seems to have a hard time determining Kabuto's thoughts at times, as seen when he orders Kabuto to kidnap Sasuke, and wonders if perhaps Kabuto will kill him instead, even giving hints that now would be an opportune time to halt his plans. Even in this instance, Orochimaru has great confidence in Kabuto's loyalty, often laughing at the prospect that Kabuto would betray him.

While Kabuto does generally obey Orochimaru's orders, even going so far as to save Orochimaru's life at times, he is not above helping Orochimaru's enemies; for example, he heals Sakura Haruno and Hinata Hyuga on different occasions. When healing Sakura, Kabuto hints at an ulterior motive, saying that Akatsuki is the true enemy; Orochimaru later says something similar, pointing out that the death of more Akatsuki members means fewer enemies for him. He is also generally polite to Naruto Uzumaki despite the fact that they have conflicting motives during their encounters. Despite his unclear goals, Kabuto seems to be content in following Orochimaru, using the alignment to make requests for specific bodies with which to further his knowledge.

Chunin Exam arc
Prior to the first phase of the Chunin Exams, Kabuto befriends the members of the Rookie Nine with information on their fellow competitors. During the second phase of the exam, Kabuto joins forces with Team 7 in an attempt to finish the phase with little hassle. When the group encounters Team Oboro, Kabuto's glasses are knocked off of his face after saving Naruto from an attack. This causes Kabuto to momentarily lose his cool and reveal his Killing Intent, paralysing both Team Oboro and Sasuke and Sakura. This gives Naruto Uzumaki the opportunity to defeat the Rain Genin. Upon completing the second test, Kabuto reunited with his teammates, Yoroi Akado and Misumi Tsurugi. Back with his team, Kabuto reports to Orochimaru what he has learned about Sasuke Uchiha.

With Orochimaru now in a position to observe Sasuke firsthand, Kabuto forfeits prior to the preliminary matches of the Chunin Exam. While explaining to Naruto that he is fatigued, injured, and unwilling to risk his life further, Kabuto actually forfeits because he is afraid that prolonged fighting will cause him to show his true abilities, blowing his cover. After Orochimaru gathers enough of his own intelligence about Sasuke, he instructs Kabuto to kidnap Sasuke. He was able to take out the eight ANBU members guarding Sasuke and almost succeeded in accomplishing this task, but Kakashi Hatake intervenes before he could finish, forcing Kabuto to flee.

During the final matches of the Chunin Exam, Kabuto disguises himself as an ANBU member and mingles with the audience. When Sasuke injures Gaara, Kabuto uses his Temple of Nirvana Technique to put the audience to sleep and signal the start of the invasion of Konoha. He does not contribute in the battle, pointing out that Kakashi would only copy his abilities, and flees the village when the invasion fails.

Search for Tsunade arc
With Orochimaru being heavily injured as a result of the failed invasion, Kabuto accompanies him in searching for Tsunade, who they hope will be able to heal him. Upon finding and in time seeming to persuade Tsunade, Kabuto notices that Tsunade's medical jutsu is filled with killing intent, and as such stops her before she can kill Orochimaru. With it clear that Tsunade won't willingly help them, Kabuto fights Tsunade in the hopes that he can force her into submission. After taking a Military Rations Pill to boost his abilities, Kabuto is able to keep up with Tsunade and land a number of disabling attacks, though she eventually lands a crippling attack upon him. Upon recovering, Kabuto slits his wrists, drenching Tsunade in his blood and invoking her hemophobia, allowing him to land a series of unopposed attacks.

When reinforcements for Tsunade arrive, Kabuto is pitted against Shizune, who he quickly defeats. As he returns his attention to Tsunade, Naruto also comes to her rescue, surprised that Kabuto is a spy for Orochimaru. While Kabuto doesn't view Naruto as much of a threat, Naruto is able to hit Kabuto with his Rasengan, doing so much damage to Kabuto that the he is unable to fully heal. While Kabuto had managed to strike a fatal blow to Naruto's heart before the Rasengan hit him, Tsunade heals Naruto before he can die, leaving Kabuto to lie on the ground, defeated. He later recovers to the point of helping Orochimaru summon Manda, and flees with his master once Orochimaru is defeated.

Sasuke Retrieval arc
As Orochimaru anxiously awaits Sasuke Uchiha's arrival at their lair, Kabuto encourages him to switch to a new body while he still can, even offering his own body. Although Orochimaru rejects the offer, Kabuto finds a strong body for Orochimaru to switch to just in case. Although Kabuto's efforts seem to have been needless for a while, Orochimaru finally grows tired of waiting for Sasuke and switches to the body that Kabuto prepared. Kabuto also manipulated Kimimaro to leave to retrieve Sasuke, despite the former's terminal illness.

Sasuke and Sai arc
When Team Yamato comes to meet one of Sasori's agents, Kabuto is revealed to be that agent. Yamato, disguised as Sasori, is able to learn some things from Kabuto, though just as he is about to capture the latter Orochimaru shows up. Although it seems as though Orochimaru is about to kill Kabuto as a traitor, Kabuto cuts off the Sasori disguise's head, having intended to kill the actual Sasori under Orochimaru's orders. With his cover blown, Yamato calls to the rest of his team for assistance, and Naruto eventually starts to fight Orochimaru. At the start of the battle, Kabuto tries to attack Naruto, though he is almost instantly repelled by one of Naruto's roars.

After the battle, Kabuto joins Orochimaru and former Team Yamato member Sai in returning to their hideout, and Kabuto is tasked with leaving behind a dead body that looks like Sai to prevent anyone from following them. When Sai is later captured, Kabuto comes to his aid, only to be captured by Sai instead. Unable to escape, Kabuto is forthcoming with information, and discourages Naruto from trying to retrieve the traitorous Sasuke. Kabuto is left by himself as Team 7 searches the lair, though is at some point freed and joins Orochimaru and Sasuke when they depart.

Three-Tails Arrival arc
Realizing that the time was fast approaching for him to change bodies, Orochimaru sent Kabuto to fetch Guren. When Kabuto nears her location Guren attacks him to see if he is worthy of being Orochimaru's attendant. Satisfied, she accompanies him to Orochimaru's lair and is left in charge of testing the prisoners there and forming a team.

When Guren and her team fail to kill a group of Konoha ninja, Kabuto has them help in summoning the Three-Tailed Beast. Kabuto ordered Guren to restrain it with her Crystal Release. When she proves unable to do so, Kabuto retreats upon realizing he needs to rethink their strategy.

Later he begins preparations for a final attempt to capture the Three-Tails; during which he secretly meets with Rinji who is revealed to be a spy for Kabuto. After learning that the Hidden Leaf has begun attempting to seal away the Three-Tails, Kabuto retrieves Guren and Yukimaru to stop them.

While successfully stopping the Leaf Ninja from sealing the Three-Tails, they manage to obtain Yukimaru, much to Kabuto's dismay. Deciding to take desperate measures, Kabuto tricked Kigiri, Kiho, and Nurari into thinking they were dying from the injuries they sustained from the Leaf Ninja and convinced them to go through a grueling operation of having cursed seals forcibly activated. He then disguises himself as Rinji, tricks Yukimaru to use his power to unleash the Three-Tail's full might despite knowing how weakened Yukimaru was. After a failed attempt to have Yukimaru make the Three-Tails go on a rampage, Kabuto disguised as Rinji reveals to Yukimaru that it was Guren who killed his mother, but is shocked that Yukimaru forgives her. He then attacks Guren and Yukimaru with scalpels, but are deflected by Naruto. One of the scalpels cuts Rinji's face which turns out to be a mask and it falls off, revealing that he is Kabuto. Since his meeting with Rinji, Kabuto has been posing as him. Kabuto and Naruto fight it out, with Naruto having improved enough to seemingly gain the upperhand over Kabuto. As Guren tries to escape with Yūkimaru, Kabuto then summons Rinji with the Dead Soul Technique to stop her. Guren fights him, only to be immobilized by his bats. Guren who made her promise to protect Yūkimaru at the cost of her life grabs Rinji from behind and sacrifice herself to kill him. Yūkimaru then screams in agony at what appears to be the loss of Guren. The screaming once again awakens Yūkimaru powers and causes the Three-Tails to breaks out of the seal and go on a rampage. Kabuto then leaves the scenery satisfied with what was happening.

Hunt for Uchiha arc
After Orochimaru's defeat at the hands of Sasuke, Kabuto integrates remains from Orochimaru's body into his own. The remains start taking control of his body, and Kabuto is in a constant struggle to keep them from spreading. When he later confronts Naruto, Kabuto explains his reasons: he has always defined himself as someone's underling, never living for himself. By taking the remains of Orochimaru he can become strong enough that he need never serve anyone ever again. This being an emulation of Naruto's struggles with the Nine-Tailed Demon Fox, Kabuto gives him a book containing Orochimaru's information on Akatsuki in thanks. He departs before he can be apprehended, noting that he will someday fight Naruto after he avenges Orochimaru by killing Sasuke.

Pain Invasion Arc
Sai, Yamato, and Anko are later seen tracking Kabuto. They managed to locate where he is hiding, but they were forced to give up on the mission when Yamato sensed the Demon Fox within Naruto awakening.

Shippūden the Movie 2: Bonds
Kabuto makes a small appearance along with Orochimaru in the second Shippūden movie. He tells Sasuke how Konoha been attacked, at which Sasuke told him he doesn't care. Orochimaru then ordered him to get a scroll and capture Shinnou. In the end of the movie, Sasuke failed to bring Shinnou, at which Kabuto was very shocked.

Other media
Kabuto plays a role in the Story Mode of Naruto: Clash of Ninja Revolution 2. He first appears as a shaded figure talking to Bando about his and Kagura's plot to destroy Konoha. When Bando questions Kabuto's motives in the plan, Kabuto simply replies that he has been rather bored lately and is just in it for kicks. Kabuto witnesses Kagura's defeat at the hands of Tsunade and attempts to flee only to be pursued and attacked by Naruto. During the fight Naruto defeats Kabuto with his Rasengan. Accepting defeat, Kabuto escapes, assuring Naruto that their next encounter will end differently.

Abilities
Kabuto has proven himself to be a skilled and cunning ninja, particularly because of his medical abilities. His skills are said to rival those of Kakashi Hatake on more than one occasion, and he has likewise been shown to be able to kill ANBU members with relative ease, even mockingly stating he could handle at least 10 ANBU members. During his battle with Tsunade, she remarks that Kabuto's senses and abilities might have surpassed her own when she was in her physical prime. Through keen observation, Kabuto can quickly determine the situation at hand. Even when up against a stronger opponent, he can take advantage by quickly exploiting their weakness.

He has also show himself to be a very skilled manipulator. For years, he was able to convince the entire Leaf Village into believing he was a genuinely kind-hearted man with inept shinobi skill outside of medical skills (to which he made it appear he was only decent at) to make it easier in gathering information. He is also able to use his strong powers of deception to fool his targets into aiding him in several manners.

Despite his age, Kabuto has demonstrated tremendous skill and ingenuity in medical ninjutsu. With his extensive knowledge of medical ninjutsu and of the human body, Kabuto can quickly adjust to any damage done to his body and severely injure a person with a single chakra-infused strike to the chest. He has also developed the unique ability to be able to reactivate dead cells and grow new ones, as well as being able to reanimate corpses. This is primarily convenient when he is experimenting with test subjects, though they have been used briefly in escape situations, and may be used in combat.

While stating to have lesser skill in combat, Kabuto has shown proficient enough to fend off attacks from multiple opponents and even hold his own against the superior skill of Tsunade. His style of battle revolves mainly around his medical knowledge. He will either utilize medical ninjutsu to systematically take down his opponent at key points in the body or strike at pressure points in the form of locks to subdue his opponent. Kabuto has also briefly shown a black knife of sorts during two combat situations, and has been shown to use scrolls to transport corpses.

Trivia

 * "Kabuto" is the last part of the Japanese name for aconite, a plant which can either alleviate pain or kill.
 * A kabuto is the name for a samurai's helmet or headplate. With his former teammates, Yoroi ("armor") and Tsurugi ("sword"), they form an armor-helmet-sword trio.
 * His family name, Yakushi, is the Japanese name of the Medicine Buddha, Bhaisajyaguru.
 * According to the Third Databook, Kabuto wants to fight Naruto and Orochimaru.

Quotes

 * (To Naruto) "You've thrown it all away! Dreams don't come true when you're dead!"
 * (To Naruto) "Hehehe..., you can't beat me. You have no skills, unlike Sasuke."
 * "With Orochimaru-sama now integrated into this body, I plan on finding a new me. One stronger than Orochimaru-sama himself!"