Itachi

Uchiha Itachi is one of the last of the Uchiha clan, he graduated from the chuunin selection exam at the age of 10, and when he was 13 he already became an ANBU Captain. Together with his brother, Uchiha Sasuke, they're supposably the only ones still alive after the Clan's massacre. Their clan, once praised as the strongest clan in Leaf village, was viscously wiped out by none other then Itachi himself. Itachi, a genius praised as the clan's future, killed all his family in a single night. Everybody except for Sasuke, he was left alive in order to avenge the clan. As Itachi himself stated: "At the moment you aren't even worth killing. My foolish little brother. If you want to kill me, despise me, hate me, live a loathsome life and cling to life. And then some day, when you have the same eyes as I do, come before me".
Itachi, being an Uchiha member, has the feared Sharingan bloodline ability. An eye-technique used to copy an opponent's techniques, and see through their movement. Itachi, having fully mastered the "Sharigan Eye", is able to use the clan's ultimate eye-technique: "Mangekyou sharigan".
After the incident Itachi left the Leaf, and became a missing-nin. The reason why Itachi let Sasuke live and revealed the Mangekyou Sharigan's secret is unclear. It may be because only a Mangekyou sharigan user can kill him, and he wished to be killed by his brother. Or perhaps he really let him live because he thought he wasn't even worth killing. Either way, he forced his brother to live a painfull life, swearing that he will avenge the clan and kill Itachi. The reason why he murdered all members of the Uchiha clan is also not known. Although he said it was to test his ability, it's strange that Itachi already seemed to have joined Akatsuki prior to the killing. Itachi also informs Sasuke about a second Mangekyou sharigan user besides himself.
Itachi is currently a member of the organization "Akatsuki", an organization comprised of "Sly S-Rank Criminals" as Kakashi stated. Their ultimate goal is unknown, but we do know they're after the jinchuuriki and Itachi and his partner Kisame were ordered to track down the Kyuubi (The Nine Tailed Fox Demon) jinchuuriki, and take him back with them. In other words, they were ordered to kidnap Naruto, the host in which the Kyuubi was sealed. Some time after Itachi joined Akatsuki Orochimaru tries to take over Itachi's body, but fails and ends up having his hand severely damaged. After this incident Orochimaru leaves the organization.

Itachi first appeared in episode 80, where we breifly veiwed him and Kisame observing the ruined Leaf village, after Orochimaru's invasion. Itachi and Kisame get detected by Kakashi who is defeated by Itachi quite easily using the Mangekyou Sharingan. Kakashi being defeated so easily, searches for a way to counter the Mangekyou Sharigan. We have yet to see Itachi's true power, but considering his Sharingan and the fact that he's very skilled with genjutsu and ninjutsu makes him a force to be wreckend with.