Itachi was the elder brother of Sasuke Uchiha and one of the most powerful members of the Akasuki, being a prodigy of the Uchiha clan and a Mangekyo Sharingan user. He was first introduced as an antagonist and the partner of Kisame Hoshigaki, who killed every member of the Uchiha clan but Sasuke, sparing him only for a hoped future fight. He was, however, later revealed to have been on Konohagakure's side all along and that he killed the Uchiha clan to stop them from attempting to take the seat of power in Konoha so as to prevent a civil war in the village that would have allowed its enemies to invade on the lack of order and cause the deaths of many innocents as well as a world war, and that he really spared Sasuke because he was too young to take part in the uprising and therefore had no knowledge of it and was innocent and because Itachi loved Sasuke too much to kill him anyway. He left the village and joined the Akasuki as a spy after the Third Hokage, Hiruzen, promised to protect Sasuke when Itachi wouldn't kill him and disobeyed orders from the Konoha elders. Itachi, however, also threatened Danzo, one of the elders, that if he dared to harm Sauske, Itachi would reveal everything he knew of Konoha's secrets to enemy countries in order to ensure his brother would live to kill him, wanting to make him a hero when he did.
In part 1, Itachi first appeared in Konohagakure again after the death of the Third Hokage to warn Danzo Shimura that he was alive and not to harm Sasuke under the cover that he was looking for Naruto. After giving Danzo his warning, Itachi and his partner, Kisame, continued to search for Naruto, the host of the Nine-Tailed Demon Fox, defeating Kakashi Hatake in a forced fight along the way, before finding Naruto in the company of Jiraiya, one of the Sannin. Jiraiya saw through Itachi's attempt to fool him with a pretty woman he had put under a genjutsu, returned in time to prevent them from taking Naruto, and drove them away.
In part 2, Itachi first appeared to keep Team Kakashi and Chiyo, an elder of Sunagakure, busy by manipulating a specially-made clone created by the Akasuki's leader, Pain, that had 30% of Itachi's chakra while he and the rest of the Akasuki finished removing and sealing the One-Tailed Shukaku out of Gaara, but it was defeated. However, they still managed to complete the sealing of the beast before the hide-out was reached.
Later, while being searched for, Itachi met with Naruto Uzumaki again, this time to just talk and, after he answered some questions Itachi had for him, Itachi gave him some of his power, which belonged to Itachi's long dead best friend, Shisui Uchiha. Itachi later met with Sasuke at the Uchiha hide-out where they had their final battle. Each used their powers to the full extend, Itachi proving too much for Sasuke to defeat and managing to eternally incapacitate Orochimaru, one of the Sannin, who had been defeated and absorbed by Sasuke prior.
As Itachi cornered Sasuke, instead of reaching for his eyes, he poked his brother's forehead and sealed the Amaterasu in him before finally dying with a smile and collapsing dead. He was later revived by Kabuto Yakushi during the Fourth Shinobi World War along with the other deceased Akasuki members as military weapons, but managed to escape from Kabuto's control, ultimately team up with Sauske, who had learned the truth about Itachi and forgiven him, and defeat Kabuto with Izanami, allowing him to force Kabuto to release the Edo Tensei. Itachi then showed Sasuke the full truth about the Uchiha clan massacre in a flashback genjutsu as he began to feel the effect of Edo Tensei's release. Before disappearing along with most of the other revived shinobi, Itachi told Sasuke that even though he couldn't change his brother's mind about wanting to destroy Konohagakure, which he had sworn revenge on after learning the truth about Itachi, he would love him forever.
Personality
Itachi at first appeared to be very evil and cruel, but was truly a very kind and loving ninja, who betrayed his clan out of love for his village and would not kill his little brother, who he valued even more than Konohagakure. Itachi was a very pacifist man, and usually did not take part in battle, only fighting when he had too and usually in self-defense. He was one of the few Akasuki members who got along well with their partners. Itachi was always careful not to underestimate his opponents, shown in his praise of Jiraiya saying the best he could do against the Sannin would end in a stalemate. He also had great emotion control and kept his composure even when surprised.
Abilities
Itachi was an extremely powerful ninja, able to easily defeat several other very powerful ninja, including Kakashi Hatake. He was a master and heir of the Sharingan and one of the few Uchiha who have awakened the Mangekyo Sharingan. Itachi was a master of all his clan's battle styles and serveral more and could easily outmatch most any opponent and any other Uchiha except for Madara Uchiha. Itachi had all the same Mangekyo Sharingan techniques as Madara, though his Susanoo was weaker than Madara's in every way. However, his Tsukuyomi and Amaterasu were always potent and his Susanoo's Sword of Totsuka could seal away anything it stabbed forever in a genjutsu world of drunken dreams. Using it, he incapacitated Orochimaru, one of the Sannin, in one attack. He could also use Susanoo's Yata Mirror to repel most any attack, making him nearly invincible when he used Susanoo.