Itachi Uchiha

Itachi was born into the Uchiha clan and 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 out shined all others. However, not even his family, friends, or teachers could understand him. When he was only 4, Itachi had already witnessed countless lives lost due to the Third Great Shinobi World War, which emotionally traumatized him and made him into a pacifist. At age 7 he graduated from the Ninja Academy after only one year at the top of his class, mastered the Sharingan at age 8, passing the Chunin Exams at age 10 and becoming ANBU Captain at age 13. According to Madara Uchiha, Itachi also had a lover, whom he killed during the Uchiha clan massacre.
A Young Itachi With His Family. From left to right: Fugaku Uchiha, Sasuke Uchiha, Mikoto Uchiha, Itachi Uchiha
He was praised highly by his father, who saw the family's future in Itachi and neglected his other son - Itachi's little brother, Sasuke. However, during this time Itachi showed great affection towards his little brother, Sasuke, and even encouraged his father to come to the Academy when Sasuke was enrolled, something their dad had otherwise forgotten, by saying that he would otherwise not come on a mission.
When the Uchiha began planning to overthrow Konoha, Itachi, as an ANBU, was tasked with spying on the village. Knowing that a coup d'etat 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. After this he began to act strangely, not coming to the clan meetings, speaking out against the clan, and becoming the prime suspect for the murder of his best friend, Shisui Uchiha. 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, Danzo 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. 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 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, redeeming the Uchiha name. He tortured Sasuke with his Mangekyo Sharingan and encouraged him to get 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.
Knowing that Konoha would still want to eliminate Sasuke, Itachi pleaded to the Third Hokage to keep Sasuke safe. He also threatened Danzo to not harm Sasuke or risk him revealing everything he knew about Konoha's secrets to enemy countries. Sasuke's safety was secured and Itachi returned to the original plan by becoming infamous for his slaughter of the Uchiha. He joined Akatsuki so that he could keep an eye on it and 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 arm as punishment, forcing Orochimaru to flee the organization. Sometime after joining Akatsuki, Itachi later helped recruit Deidara, Orochimaru's replacement by beating him in battle, to which Deidara swears vengance against Itachi.