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 and the partner of Kisame Hoshigaki.
Background
Itachi was born as the eldest son of Fugaku Uchiha, the leader of the Uchiha clan. He grew up during the Third Shinobi World War and became a pacifist due to what he saw then. He grew up to be the pride of the clan, graduating from the academy after one year, mastering Sharingan by the time he was eight, becoming a Chunnin at ten, joining the Anbu a six months later, and becoming an Anbu captain at age thirteen. When the Uchiha clan plotted a coup to take over Konohagakure, Itachi, who was to be their go-to, betrayed them, knowing that if they were to revolt, many innocents would die and another world war could start if Konoha's enemies were to invade on the civil war that would happen in Konoha should the coup be attempted, and supplied the village with information on the Uchiha, later buying time for the matter to be resolved. He also secretly made a deal with Danzo, one of the elders, who lied to him and promised to spare Sasuke Uchiha if Itachi would wipe out the rest of the clan should Hiruzen, the Third Hokage, fail to end their coup. After Hiruzen eventually failed to resolve the matter peacefully, the elders of Konoha ordered Itachi to wipe out the Uchiha clan. He and Tobi, whom Itachi had met prior, wiped out the Uchiha, sparing only his Itachi's brother, Sasuke. Not wanting Sasuke to hate their clan, Itachi lied to him about why he did this deed and said he was sparing Sasuke only for a hoped, future fight. He, however, 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. Itachi left the village and joined the Akasuki as a spy after the Third Hokage, Hiruzen, promised to protect Sasuke when Itachi saw through Danzo's deception about promising to let him live. Itachi, however, also threatened Danzo 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.
Series Involvement
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 temporarily seal 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 via Shisui's eye after meeting with Naruto, seal the revived Nagato with the Sword of Totsuka, and 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 loved even more than Konohagakure. Itachi also dearly loved his parents as can be seen when he spared them until last out of the Uchiha on the night of the massacre and was brought to tears and shook tremedously before he killed them. 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 several 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 most 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.