A genius above all others, Itachi was born into the Uchiha clan and praised by all as the best of his generation. However, not even his family, friends, or teachers could understand him.
Not much is known about Itachi's childhood except that when he was only 4, Itachi had already witnessed countless lives lost due to war, which emotionally traumatized and affected his demeanor. It is also known that at age 7 he graduated from the ninja academy after only one year at the top of his class. He also graduated from the Academy and mastered the Sharingan at age 8, passing the Chunin exam at age 10 and becoming ANBU Captain at age 13. According to Madara, Itachi also had a lover, whom he killed during the Uchiha clan massacre.
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.
After having acted strange for a long time, not coming to the clan meetings and refusing to train with Sasuke, it would finally seem that he snapped. With the help of Madara's teachings (and possibly the help from Madara himself), he slaughtered his entire clan overnight (with the exception of Sasuke). Due to the fact that Sasuke was never to know about the Uchiha's attempt at a coup d'etat and the Konoha higher ups's orders, Itachi used his Mangekyo Sharingan to capture Sasuke in a Genjutsu and show him how he had killed everyone. After this Sasuke ran away, but Itachi confronted him again and explained that he did it all to test himself. He also explained that the new eyes he had acquired could also be acquired by Sasuke and all he had to do was to kill his best friend. Telling Sasuke that he let him live because he had the potential to become as strong as him, he also tells him to hate him and detest him, if he ever wanted a chance of defeating him.
Itachi is a mysterious character and often acts as though he is concealing his true self. While flashbacks to his past show that he was a compassionate brother and person, his later acts and claims make it seem that this was mostly just an act. But, surprisingly, he actually is the compassionate brother he once claimed to be, but he hides this to protect his brother. When he openly challenged the clan's ideas, he showed some degree of anger but never lost his composure.
As a member of Akatsuki, he possesses incredible self control of his emotions. He constantly displays an emotionless personality, though at times, he appears to have a slight degree of anger or frustration on his face. He only shows surprise when opponents prove more powerful or skilled than he had expected, and even then he doesn't lose his composure. He does not possess any arrogance, and fully justifies any statements he makes about his power, and even compliments his opponents at times on their abilities.
In addition to having tremendous natural talent, he has also shown equally impressive intelligence. Itachi can be considered an very keen minded individual from how adept he is at thinking things through. He is rarely surprised or caught off guard by anything and also diplays great intuition as he never does anything without a reason behind it. In addition, he has also shown to be a very observant and analytical person from how quickly he can determine his opponent's tactics. Despite his status as a wanted criminal and having murdered a significant portion of his family, Itachi has no enjoyment of violence or combat, instead preferring to avoid battle or, if this is not possible, end it as quickly as he can.
Itachi's only initial interest is in Akatsuki's goals and his only loyalty appears to be to the organization and its members, putting the organization's success and secrecy before everything else. As such, Itachi gets along with his partner, Kisame, much better than most other Akatsuki members do with their own partners. Kisame appears to have a deep loyalty to Itachi, immediately doing as he instructs and constantly looking out for his well-being. Itachi treats all of his fellow Akatsuki members to this same, more respectful personality, although his courtesy is rarely returned by the other members.
However, when he is in a Genjutsu duel during his fight with Sasuke, he agressively and violently claims he only let Sasuke live so he could take Sasuke's eyes and obtain his own Eternal Mangekyo Sharingan, and then he claims the Uchiha clan is nothing but tainted and evil since many members would kill their friends to gain the Mangekyo Sharingan, and take another Uchiha's eyes to make its power permanent. Once the Genjutsu is released, he calls it his true self, implying his calm, emotionless personality was also a facade.
Madara, however, later reveals that this is not the truth. Itachi really was the caring brother that he once claimed to be only an act. He had killed his clan only because he had been ordered to by Konoha to stop the Uchiha from rebelling against the village or else Konoha would order other ANBU to kill the clan instead. His childhood experience in war had caused him to greatly fear the violence the Uchiha could cause, so he made the incredible sacrifice, and decided to murder his entire family, creating the image of his "evil" side that so many had known him to be. Deep down, Itachi had cared a lot for Sasuke, so he never revealed the treachery of the Uchiha clan to keep his brother believing their clan was a noble one. Itachi had always planned to die by Sasuke's hands, so he could compensate for all he had done by making Sasuke's name known as the hero who defeated the one who slaughtered the Uchiha clan. The reason why Itachi didn't kill Sasuke was because he loved Sasuke more than the village.