Mangekuo

Being the youngest son of Konoha Military Police Force captain and Uchiha clan head Fugaku Uchiha and his wife Mikoto and named after the father of the Third Hokage, Sasuke grew up in the shadow of his older brother, Itachi Uchiha. Itachi was an acknowledged prodigy who was viewed by the Uchiha clan as a genius, and whose work would further strengthen the bonds between the village and the clan. Sasuke's father in particular took an active interest in Itachi. Itachi, on the other hand, was rather ambivalent and full of oddly conflicting undercurrents, acknowledging Sasuke by taking interest in his progress as a ninja when even their father did not, but spending little quality time with Sasuke. Even after Sasuke enrolled in the Ninja Academy, he was still unable to escape Itachi's shadow. Despite consistently scoring at the top of every class, Sasuke failed to receive any recognition from his father, who defined Sasuke's success only in terms of his older brother. Itachi acknowledged the rivalry Sasuke felt, telling him, "We are brothers. I am the wall you must climb over, so you and I will continue to exist together."

Itachi's relationship with his father deteriorated further after Itachi was questioned by other members of the Uchiha clan regarding the apparent suicide of his best friend, Shisui. As Itachi's behavior became increasingly bizarre, Sasuke's father began spending more time with Sasuke, and began teaching him the Great Fireball Technique, a common family jutsu seen as a sign of coming of age in the Uchiha clan. When Sasuke first tried to perform the jutsu, he only had a small flame come out and after that, some smoke. His father said that Itachi was able to do it on his first try at Sasuke's age, which disappointed him. Later, with one week of training, Sasuke (age 7 at the time) performed the jutsu in front of his father, proving himself a genius as well in his own right and finally receiving his father's recognition. Sasuke's father told him that he could now proudly wear the clan's emblem on his back. Following this, Sasuke also received a stern warning not to follow in Itachi's footsteps any further, surprising him as before, he was told to be like him; and his father showing he was proud of Sasuke for the first time the same way as he used to with Itachi.
Sasuke activates his Sharingan for the first time.JacceAdded by Jacce
Shortly thereafter, Itachi massacred every member of the Uchiha clan, sparing only Sasuke, saying that he was not even worth killing. In addition, Itachi told him, "You've always wanted to surpass me. That's why I'm going to let you live... if you wish to kill me, then hate me, despise me... and by all means flee, cling to your wretched life." Itachi also revealed to Sasuke where the secret meeting place of the Uchiha was hidden, and suggested that Sasuke go there to learn what the true history and purpose of the Sharingan was. He told him that "to awaken the Mangekyō Sharingan," he "had to kill the person he was closest to", and that "when we battle, we should have the same eyes". Sasuke thought he had fainted after this, but later remembered that he had managed to stand back up, awaken his Sharingan, and pursue Itachi. Upon reaching Itachi, he managed to knock off his forehead protector. Itachi was seen shedding a tear as he was picking up the forehead protector. He then proceeded to incapacitate Sasuke, who awoke in the hospital remembering only that which Itachi wanted him to.

Itachi was born into the Uchiha clan and was praised by all as the best of his generation - a genius above all others. Even by the standards of the battle-adept Uchiha clan, Itachi's prowess far outshone them. However, not even his family, friends, or teachers could understand him. When he was only four, Itachi had already witnessed countless lives lost due to the Third Great Shinobi World War, which emotionally traumatized him and turned him into a pacifist. At age 7, he graduated from the Ninja Academy at the top of his class after only one year, mastered the Sharingan at age eight, passed the Chūnin Exams at age 10, and became an ANBU Captain at age 13.
He was praised highly by his father, who saw the family's future in Itachi, hence neglecting his youngest son, Sasuke. However, during this time Itachi showed great affection towards his little brother and even encouraged his father to come to the Academy enrollment ceremony for Sasuke, something their father had otherwise forgotten, by saying that he would not go to an important mission that would establish Itachi a place in the ANBU ranks, otherwise.
A young Itachi with his family. From left to right: Fugaku, Sasuke, Mikoto, Itachi.AlienGamerAdded by AlienGamer
When the Uchiha began planning to overthrow Konoha, Itachi, as an ANBU, was tasked with spying on the village. Knowing that a coup d’état would only lead to another Ninja War; Itachi began spying on the Uchiha instead, passing on information to the Third Hokage and the other Konoha elders. During this time he began to act strangely, not attending the clan meetings, speaking out against the clan, and becoming the prime suspect for the murder of his best friend, Shisui Uchiha. Due to this the clan lost hope in Itachi and his father began shifting his focus to Sasuke instead. Meanwhile, the Third's attempts to negotiate a bloodless end to hostilities between the two parties were proving ineffective so, against the Third's wishes, Danzō Shimura and the Konoha Council ordered Itachi to wipe out the Uchiha.
Before the fateful night arrived, Itachi discovered the existence of Madara Uchiha, who was attempting to rekindle the flames of war in Konoha. In return for not attacking Konoha, Itachi offered to help Madara claim vengeance against the Uchiha for turning their backs on him decades earlier; Madara agreed. Itachi slaughtered his entire clan overnight, sparing only Sasuke as Sasuke's life was worth more than the village to Itachi, and he could not follow through with the orders to kill him as well. Instead, he made himself out as a villain, who had killed their innocent family only to test his abilities. By doing so, he wanted Sasuke to take vengeance on him and become a hero to Konoha, thus redeeming the Uchiha name. He tortured Sasuke with his Mangekyō Sharingan and encouraged him to become strong enough to kill him and avenge their family. Hoping that his beloved little brother would someday be powerful enough to kill him, Itachi left the village as a traitor. According to Madara Uchiha, Itachi also had a lover, whom he killed during the Uchiha clan massacre.[4]
Itachi massacres the entire Uchiha clan.Magatama90Added by Magatama90 Knowing that Konoha would still want to eliminate Sasuke, Itachi pleaded with the Third Hokage to keep Sasuke safe. He also threatened Danzō to not harm Sasuke or risk him revealing everything he knew about Konoha's secrets to enemy countries. Sasuke's safety was therefore secured and Itachi returned to his original plan of becoming infamous for his slaughter of the Uchiha clan.
Itachi joined Akatsuki in order to keep an eye on the organization, along with Madara Uchiha, in case either ever acted against Konoha, all the while planning for his death at Sasuke's hands. While a member of Akatsuki, Itachi was approached by Orochimaru, who tried to steal his body to gain the Sharingan. Itachi trapped him in a genjutsu and severed his left hand as punishment, forcing Orochimaru to flee the organization. He was partnered with Kisame Hoshigaki, who he initially greeted with threats after the latter tried to intimidate him. Eventually, they became good partners for sharing a common bond of having betrayed the people closest to them. Sometime after joining Akatsuki and forcing Orochimaru to flee, Itachi helped recruit Deidara, by beating him in battle using only his genjutsu, to which Deidara swore vengeance against Itachi.
At some time, Itachi contracted a fatal illness, to which he kept himself alive through the intake of various medicines, in the hopes of living long enough to die by Sasuke's hands.

One day when they were younger, Sasuke accompanied Itachi on one of his training sessions. Impressed by Itachi's skills, Sasuke asked Itachi to teach him something, but Itachi, as always, refused by poking him on the forehead and promising to help him later. As they returned home, Sasuke marveled at the Uchiha clan's accomplishments as explained to him by Itachi. When they encountered their father, Fugaku, he spoke to Itachi about an approaching mission. Itachi refused to take part, wishing to take part in Sasuke's Academy orientation. Seeing that Itachi was determined, Fugaku volunteered to go in his place.

Ever since his first appearance, Itachi had always been mysterious, and had acted as though he was hiding his true self. While flashbacks to his past showed that he was a compassionate brother and person, his later acts and claims made it seem that this was mostly just an act. But, surprisingly, he genuinely was the compassionate brother he had once claimed to be, although he hid this from the rest of the Akatsuki to protect Sasuke.

As a member of Akatsuki, he possessed incredible self-control of his emotions, constantly displaying an emotionless personality. He only showed surprise when opponents proved more powerful or skilled than he had expected, and even then he didn't lose his composure. He did not possess any arrogance, and fully justified any statements he made about his power, even complimenting his opponents at times on their abilities.
In addition to being an extremely powerful ninja, Itachi had equally impressive intelligence, and was shown to be very wise. He was exceedingly sharp and observant to any situation at hand, as he was rarely surprised or caught off guard. He also displayed great intuition, as he was almost never deceived and even then almost instantly realized the truth. Despite his status as a wanted criminal, and having wiped out his clan, Itachi took no enjoyment in violence or combat, instead preferring to avoid battle or, if this was not possible, end it as quickly as he could.
Itachi's only initial interest was in Akatsuki's goals, and his only loyalties appeared to be to the organization and its members, seeming to put the organization's success and secrecy before everything else. As such, Itachi got along with his partner, Kisame, much better than most other Akatsuki members did with their own partners. Kisame appeared to have a deep loyalty to Itachi, immediately doing as he instructed and constantly looking out for his well-being. Itachi treated all of his fellow Akatsuki members to this same, more respectful personality, although his courtesy was rarely returned by the other members.
However, when he was in a genjutsu duel during his fight with Sasuke, he cruelly claimed he had only let Sasuke live so that he could take Sasuke's eyes and obtain his own Eternal Mangekyō Sharingan, after which he then claimed the Uchiha clan had been nothing but tainted and evil, since many members would kill their friends to gain the Mangekyō Sharingan, and would take another Uchiha's eyes to make its power permanent. Once the genjutsu was released, he called it his true self, implying his calm, emotionless personality was also a façade. This persona was later revealed to be only an act to ensure that he would die by Sasuke's hands, his love for his younger brother being the fundamental aspect of his personality.
In the end, Itachi put all of his faith in Naruto, who he notes as the person most determined to save and protect Sasuke. After hearing Naruto's answer to keep protecting Sasuke and everyone in Konoha, Itachi gave him some of his power. He was very grateful towards Naruto's determination to save Sasuke, and was smiling after hearing Naruto's response. Itachi even trusted him enough to bring Sasuke back to the light.