Kabuto Yakushi was an S-rank Missing-nin of Konohagakure and the right-hand man of Orochimaru. In part 2, after his master's defeat, Kabuto later became the partner of Akatsuki leader Obito Uchiha during the Fourth Shinobi World War.
The village Kabuto was born in was destroyed when he was very young, and he was later found by Nanno Yakushi, an orphanage owner and retired kunoichi, who gave Kabuto her glasses when he was in need of a pair, and he became her son. Nanno taught Kabuto how to use medical ninjutsu, which he managed to help the orphanage with by gaining new funds. One day, he treated a Konohagakure Root Anbu named Orochimaru, who asked him if he ever thought of being a shinobi, to which Kabuto declined. Nanno was later visited by her former superior, Danzo Shimura, one of Konohagakure's elders, who wanted her for a mission. He also offered funds in exchange one of the orphans, to which Kabuto accepted. Throughout the years, Kabuto and Nanno, who was regularly shown fake photographs of Kabuto to keep her working for Danzo, became effective agents of Root, eventually causing Danzo to become wary of their skill and potential to betray him, which caused him set Nanno up to kill Kabuto. Though he killed her, he is hurt when she couldn't recognize him when he tried to heal her, and later encountered Orochimaru again, who was to kill the one of them that lived. However, instead, he took Kabuto under his wing, promising to protect him from Danzo and giving him a new identity as a captain of the Konohagakure Medical Corps' adopted son in exchange for his loyalty and help. Kabuto went on to become his right-hand man in the village he founded called Otogakure, and for years helped him with extensive research which furthered his knowledge of the human body. After orochimaru joinded, but left the criminal organization of Akatsuki, Kabuto was sent to spy on the organization, becoming the seeming servant of Sasori, who put him under his influence with a unique technique and sent him to spy on Orochimaru, who released the technique, allowing Kabuto to continue to act as Sasori's servant on Orochimaru's behalf to gain information on Akatsuki. At the time Orochimaru decided to destroy the village of Konohagakure, Kabuto enrolled in the Chunnin Exams, which he had done regularly throughout the last few years, to have a cover.
Kabuto made his debut in part 1 of the series in the testing room of phase one of the Chunnin Exams in Konohagakure, giving Sasuke Uchiha amoung others information on other shinobi, before Ibiki Morino appeared and gave them their test. In the second phase, Kabuto helped Sasuke and his teammates, Sakura Haruno and Naruto Uzumaki to get through the forest, showing them a shortcut and helping them get the scroll they needed to pass. As so many Gennin had survived the second phase, a preliminary fight round was decided, but Kabuto quit the Exams before it began. He later went to the hospital to get Sasuke Uchiha for his master, Orochimaru, killing several Konoha Anbu before being found by Kakashi Hatake, who he outwitted and escape from. He later met with Baki, a Sunagakure Jonin from the village Orochimaru had recruited to help his plan to destroy Konohagakure, to hand over the plans for their side, before pointing out the presence of Hiyate Gekko, who was killed by Baki. Kabuto later appeared at the battle arena for the final stage of the Chunnin Exams, where, after the time was right, he activated a genjutsu to put the people present asleep, with only some Jonin dispelling it. After the invasion failed, Kabuto fled the village after his master and the Sound Four, his bodyguards.
With Orochimaru's arms crippled by the Third Hokage, he and Kabuto sought out his old teammate Tsunade, the beast medical shinobi alive, to heal them. After they offered their deal to her, Kabuto gave her a week to think it over, later preventing her from killing Orochimaru after recognizing she mean't to kill him after she accepted their deal. After this, they led her away from town and Kabuto battled her, managing to weaken her physically before being seemingly disabled. However, he managed to recover his bodily control and nearly strike her down, before Jiraiya, the last of the Sannin(Tsunade and Orochimaru being the others) arrived with Tsunade's assistant, Shizune and a very surrpised Naruto Uzumaki. Kabuto managed to defeat Tsunade, before summoning two giant snakes for Orochimaru. Taking on and defeating Shizune, Kabuto fought against Naruto and seemingly outmatched him, mocking his perceived lack of talent, only to be defeated when naruto mastered the Ransengan, which Jiraiya had been teaching him. Though he struck Naruto's heart and focused medical ninjutsu right where Naruto hit him, Tsunade healed Naruto and saved him with the help of his tailed beast, Kurama, the Nine-Tails, and the Rasengan had much damage to heal, causing Kabuto to collapse. He later managed to recover enough to summon Orochimaru's strongest summon, Manda, who was disabled by Tsunade, who went on to defeat Orochimaru, whereafter Kabuto fled with him, declaring they would meet again some day.
When Orochimaru sent his Sound Four bodyguards to retreive him Sasuke Uchiha, time drew too short for Orochimaru to continue waiting, and Kabuto told him he would need to transfer into someone else before it was too late, even offering his own body to suffice. Afterwards, Kabuto picked out a suitable vessel, which Orochimaru switched his soul into, later awakening his master's strongest follower, Kimimaro and sending him to aide the Sound Four. Though all five were killed, Sasuke did make it to their hideout, where Kabuto warned him to show more respect of Orochimaru, causing Sasuke to give him a frightening stare, Kabuto noting his intense chakra.
In part 2, Kabuto made his debut at a set-up meeting with one of the crminal organization Akatsuki, Sasori, where, after Orochimaru arrived and seemingly tried to kill him as a traitor, Kabuto exposed the disguised Yamato of Konohagakure from within his Wood Release transformation. Yamato then called up his team, which included Naruto Uzumaki and Sakura Haruno. After Naruto attacked Orochimaru and sent him through the trees, Kabuto tried to attack him after he reached his three-tailed Kurama transformation with his Chakra Scalpel, only to be repelled and badly hurt by the shockwave Naruto roared. After healing himself, Kabuto approached the Konoha shinobi, asserting that Sasori must be dead and, observing the battle between Orochimaru and Naruto, later healed Sakura after she had been hurt by Naruto to help his side by killing more Akatsuki, which he saw as the real enemy. He later joined orochimaru after the fight and left with him and Sai, one of Yamato's team sent by his master's old comrade Danzo Shimura, returning to their current hideout. After Sai was later captured by Yamato and his team, Kabuto saved him, only for Sai to betray and capture him. He later escaped, however, and left with Orochimaru and Sasuke. Later, when Orochimaru's time to transfer into Sasuke's body had come, Kabuto asserted his master was living on willpower alone and left to get him the strongest medicine he had to let him be as strong as possible for the transfer technique. However, in his absence, Sasuke betrayed and defeated Orochimaru, sealing him within him, much to Kabuto's intense shock. After his master's defeat, Kabuto integrated some of his DNA into his body, causing his chakra to start to take over Kabuto. He later confronted Naruto, who he told of his will to surpass his master, managing to escape capture and flee to master his new powers to kill Sasuke for betraying Orochimaru.
Kabuto was not seen again for a long while, in which he spent experimenting on himself and mastering several new powers, along with those of Orochimaru. After e fully mastered all of his various new abilities, Kabuto made his way to Obito Uchiha, the leader of Akatsuki posing as Madara Uchiha, who he attempted to convince into letting him help with the approaching Fourth Shinobi World War in exchange for Sasuke Uchiha by reviving all the deceased Akatsuki members with his newly mastered Summoning: Impure World Reincarnation. When Obito asked what would happen if he did not accept, Kabuto summoned a sixth coffin containing the real Madara Uchiha, to Obito's intense shock and horror. Though he agreed to ally with him, Obito told Kabuto that Sasuke would not be handed over until Akatsuki had won the war. Kabuto later gave Deidara a tag that allowed him some free will, but still allowed Kabuto to control if he wanted, and the two left to hunt down the Island Turtle after word got to Obito that was where the remaining jinchuriki had been sent. When they were closer, Deidara created clay clones of them which were intercepted by Onoki, the Third Tsuchikage, who himself was a rock clone. After these revealations, Kabuto forced Deidara to continue to the Island Turtle, where he ordered Deidara to distract Onoki, while he tried to capture Naruto. He, however, was captured by the alliance(Allied Shinobi Forces) shinobi present, managing to escape and capture Yamato, bringing him back to Obito after forcing Deidara back into his coffin. Though his partner, obito, questioned his failure, Kabuto explained that Yamato could help strengthen their White Zetsu Army.
After the war had commenced, Kabuto fought and defeated Anko Mitarashi, Orochimaru's former student from Konohagakure, taking her Cursed Seal of Hevean's chakra to allow him to better control the mass number of shinobi he had revived recently, despite Obito's initial order to have her killed. Kabuto began to direct the revived shinobi in various directions to several battlefields, using the previous Kage he had revived to go after the Fourth Division led by Gaara. After losing several of the revived Akatsuki amoung other revived shinobi, Kabuto had used Itachi Uchiha and Nagato to go after Naruto and Killer B, the Eight-Tails, Gyuki's jinchuriki, but the former escaped his control and sealed Nagato with help from Naruto and B, causing Kabuto to lament that he would have to use his trump card earlier than he thought. The Kage he had sent for the Fourth Division had mostly been sealed as well, as the Fourth Kazekage and the Second Mizukage had both been sealed by Gaara, and the Third Raikage was defeated by a shadow clone of Naruto, who had sent others all over the battlefield, helping to destroy a lot of the White Zetsu Army and taking out other revived shinobi. The original had helped Gaara and Onoki to try and seal the Second Tsuchikage, Mu, who was only partly sealed after splitting himself in two. Kabuto, when he could, had Mu summon Madara Uchiha to the battlefield, who confronted the Fourth Division with Mu. Asking who had revived him, Kanuto took control of Mu and talked with Madara, requesting a demonstration of his legendary power. After Madara overwhelmed and destroyed nearly the division, despite the presence of two Kage, Kabuto told Madara of his theories he had about him from the experiments he and Orochimaru had conducted, which garnered a compliment from Madara on how well prepared he was.
In response to Madara's presence and the significance of what he had done to the Fourth Division, Tsunade, the Fifth Hokage, and A, the Fourth Raikage, arrived on the battlefield, repelling Madara and Onoki in their attempted assault of the survivors of the former's technique. At first, Kabuto tried to keep Mu in the battle, though he later forced him to flee and the battle between Madara and the five Kage became more intense. Observing Madara's progress against the Kage and being very pleased with his power, Kabuto later found him confront by Itachi Uchiha, who had begun to seek him out after Nagato's sealing, who was soon joined by his brother, Sasuke. Delighted that his reward for helping in the war had come to him, Kabuto prepared for battle after the Uchiha brothers teamed up to fight him, as Itachi was there to stop the Impure World Reincarnation. The battle was long and, despite Sasuke and Itachi's combined efforts, Kabuto was able to dominate the fight for most of the battle with his mastery of Sage Mode, Orochimaru's powers, the powers of the deceased Sound Five members, as well as the powers of Sasuke's former teammates from Taka. Eventually, Kabuto overpowered them to the point that Itachi sacrificed his left eye to use the forbidden dojutsu Izanami, which trapped Kabuto's mind in a an infinite memory loop of replaying events of the battle, while leaving him physically paralyzed. Itachi then ended his technique, sending himself and all other revived shinobi besides Madara, who severed his contract with the technique to remain, back into the afterlife. Afterwards, Sasuke revived Orochimaru through Anko's Cursed Seal, and he recollected his power from Kabuto, removing his snake-like body modifications and releasing his Sage Mode, leaving Kabuto incapacitated.
Eventually, Kabuto accepts himself and all his pain, and manages to break the Izanami, now realizing who he is and that he has a home to return to. Grateful to Itachi, Kabuto goes to Sasuke's side and heals the defeated Uchiha of the fatal wound Madara had given him. While he is doing so, Orochimaru and Sasuke teammates arrive, shocked to see him. Realizing he was right to suspect it was Orochimaru's group he'd sense coming, Kabuto asked the Sannin what he was doing, the latter asking the same. Noting to Suigetsu that he didn't things like he used to anymore, Kabuto told Karin he had no intention to take Sasuke, stating he had saved him using his studies of Hashirama Senju's cells and his medical ninjutsu abilities. Confirming to Orochimaru that he had broken out of Itachi's Izanami, he states that Itachi had helped him realize his mistakes, noting that he'd like not to lose home he had and that Itachi's resolve had become his own. Admitting Suigetsu was right about him and Obito being the ones to start this war, Kabuto notes they had both lost their place in this world, but that he now knew who he was and what he had to do. With Madara's successful activation of the Infinite Tsukuyomi later, Kabuto likely fell prey to it along with everyone else and was tethered to the Shinju by the mighty ancestor to let it reclaim its chakra from him. After it was eventually released, He was set free and probably returned to the orphanage he lived in as a child.
As a child, Kabuto, due to his amnesia, was very shy, quite, and kind. After he was recruited by Danzo Shimura as a Root agent, however, he changed some, though his love for his adopted mother, Nanno Yakushi, was still in place. After Orochimaru recruited him, Kabuto became extremely loyal to the Sannin, to the point of offering himself to him as a new vessel after Sasuke Uchiha didn't arrive in time. Orochimaru had an unquestioning faith Kabuto's loyalty to him, and laughed at the notion of Kabuto ever betraying him. Kabuto could, however, be hard to determine, as Orochimaru sometimes had trouble predicting his thoughts. Kabuto had a dry sense of humor, but was always respectful and polite to others most of the time, espescially Naruto Uzumaki, who particularly interests him. But for all his loyalties to his master, Kabuto would help his enemies if he thought it was necessary, as he healed Sakura Haruno after she had been hurt by Naruto's out-of-control Kurama chakra so that she could help in killing more members of Akatsuki. Despite all his past loyalty to Orochimaru, after his defeat by Sasuke, Kabuto began to strive to surpass him, and his many conflicting statements have since pointed that he is now only loyal to himself, developing the same kind of selfishness that Orochimaru himself had. However, after his defeat by itachi and time caught in his Izanami, Kabuto came to accept himself and his pain, and had a complete change of heart, somewhat becoming like he was as a child.
Though he was only a Gennin in rank, Kabuto proved from the very start to be a very powerful shinobi with skill easily enough to be equal to a Jonin, and able to stand against a Kage. Even as a child, his skill as a Root member made him so much of a threat to the organization that its leader, Danzo Shimura, felt the need to kill him to keep him from ever betraying him. Even Orochimaru was attracted to his potential and has openly stated, as has his teammate, Jiraiya, that Kabuto's power was easily enough to equal Kakashi Hatake, one of Konohagakure's strongest shinobi. Kabuto was able to easily kill several Konoha Anbu single-handedly, stating he could have handled as many as ten at once. During their battle, Kabuto was able to the defeat the much more powerful Tsunade, one of the Sannin, along with Jiraiya and Orochimaru, despite her superior battle skills. He also easily defeated Tsunade's assistant, Shizune, and nearly defeated Naruto Uzumaki, the jinchuriki of the strongest tailed beast, Kurama, who had been training under Jiraiya, only losing to him because of Tsunade and Kurama healing Naruto's normally fatal wound and his healing abilities not being enough to heal him from the damage done by Naruto's Rasengan. After mastering Sage Mode, Orochimaru's powers, the powers of the deceased Sound Five members, and the powers of three of Taka(Suigetsu, Karin, and Jugo), Kabuto's combat prowess grew exponentially, to the point of overwhelming the combined efforts of both Itachi and Sasuke Uchiha, despite their massive respective combat abilities and the former's mastery of the Mangekyo Sharingan, as well as the latter's possession of the Eternal Mangekyo Sharingan, eventually forcing Itachi to sacrifice his left eye and the ability to use Amaterasu and Susanoo to use Izanami to finally incapacitate Kabuto and force him to end his Summoning: Impure World Reincarnation technique.
Kabuto was taught how to use medical ninjutsu from his adopted mother, Nanno Yakushi, and did it with great skill, even as a child. The fact that he could proved he had great chakra control. When he grew older, he gained the ability to use chakra scapels to disable or even kill his enemies. Using it, he weakened Tsunade's left leg and right arm muscles in an instant, enabling him to later defeat her. Using this together with taijutsu, which he claimed to be less skilled in, but still good enough to fight Tsunade, known for her incredible raw strength, Kabuto was able to easily put most opponents at a disadvantage and overwhelm them. Tsunade contemplated that Kabuto's speed, sharpness, and precision surpassed her own in her prime. Kabuto also has exceptional healing powers, able to heal from most injuries in little time, which impressed Tsunade enough to attract her focus. Kabuto had a kind of corpse theme, as he could manipulate dead bodies for any purpose and mastered the Summoning: Impure World Reincarnation beyond even Orochimaru, who completed the technique. His prowess with it was shown when he managed to revive and mantain control over many dead shinobi, including deceased Akatsuki members. Kabuto also has very large chakra reserves, able to summon two giant snakes at once while helping Orochimaru against Jiraiya, and later summon another, Manda, despite being defeated.
Kabuto also performed various experiments on himself, first integrating Orochimaru's DNA, then later DNA of the Sound Five and most of Taka, allowing him access to various new powers. He could use Orochimaru's snake powers in conjunction with Suigetsu Hozuki's liquid transformation to great effect, as well as use the special powers of Tayuya, Jirobo, Kidomaru, and Sakon and Ukon, using Tayuya's special, musical genjutsu to great effect, able to catch the Uchiha brothers, Itachi and Sasuke, in a genjutsu that neither could break out of on their own, use Sakon and Ukon's powers to dangerous effect, and use Kidomaru's webbing to entrap Sasuke and Itachi. Kabuto also used Kimimaro's Shikotsumyaku kekkei genkai to create a barrage of spikey bones, which he launched at Sasuke and Itachi, and which was so much that even Sasuke's Blaze Release couldn't completely destroy it. Kabuto could emerge a giant white snake form, just like Orochimaru could, that allowed him to swallow people whole and escape a trap, and could use Orochimaru's to attack directly and, if that failed, emerge from its mouth for a surprise attack, the doing of which allowed him to catch Itachi off-guard and bisect him. Kabuto mastered Sage Mode, which even Orochimaru couldn't attain, which caused him to take on a, as he put it, dragon-like transformation. Sage Mode greatly enhanced Kabuto's physical ailities, able to take Sasuke's sword from him before he could react, attack Itachi with it before he could react, tear off his arm easily with just the bare of his teeth, and easily dodge attacks from Sasuke's Susanoo, despite not being able to see, as Sage Mode allowed him to predict danger through feel. Kabuto also showed use of Water Release in Sage Mode, able to instantly create a large waterfall that not only countered Sasuke's Fire Release, but also swept the Uchiha away in a flood.
In terms of intelligence, Kabuto had immense knowledge of various techniques and of the human body, which greatly complimented his medical ninjutsu. He even managed to outwit Kakashi Hatake, a natual genius of a shinobi, with ease. Kabuto was able to easily regain control over his body after Tsunade reversed his nerves by correctly guessing which parts moved which, a feat that impressed the Sannin. Due to experiments he did with Orochimaru, Kabuto was able to make several correct guesses about Madara Uchiha after his last battle with Hashirama Senju, something which the legendary shinobi forefather complimented him for after his revival.