Itachi

Antroz' Summary
Itachi Uchiha is currently deceased. However, at one point in time, he was trained by Madara Uchiha, who helped him in committing parricide. He was one of the known people to have unlocked the Mangekyou Sharingan. To obtain the Mangekyou Sharingan, he killed his clan friend Shisui, and used his Sharingan to copy Shisui's handwriting, making it look like Shisui committed suicide. Itachi then continued on to kill his clan, but he couldn't kill him brother. He was able to convince the village elders to hide the truth from his brother, and became a villain all for the sake of having his brother become a hero. He eventually died of a disease during his fight with Sasuke about 9 years after committing parricide.

Wikipedia's Summary
tachi Uchiha (うちは イタチ, Uchiha Itachi?) is a former ninja of Konohagakure and the partner of Kisame Hoshigaki. Though having spent his early youth as the kindly child prodigy of the Uchiha clan, Itachi is treated as a villain for much of the series. He is initially portrayed as being solely responsible for the slaughter of his entire clan and family, sparing only his brother, Sasuke Uchiha, with the explanation that he was testing his abilities.Around the time of his death his villainous role is downplayed; not only is he revealed to have had help from his master, Madara Uchiha, in killing the clan, but he performed the massacre under the orders of Konoha's leadership so as to prevent an Uchiha coup d'état. Though he was ordered to kill Sasuke as well, Itachi could not bring himself to do so, instead forcing Konoha to keep Sasuke safe or risk Itachi going public with his mission. For his part, Itachi accepts his role as a villain, never telling anyone of who else was involved and even encouraging Sasuke to grow strong enough to kill him and avenge their family.

Itachi and the Akatsuki Organization

Throughout the series Itachi attempts to avoid combat, seeking nonviolent solutions instead. If he is forced to battle he usually remains stationary, only acting in self-defense. From his idle position he studies an opponent's movements with his Sharingan eyes, trapping them in illusions when opportune by merely pointing or looking at them. Itachi at times uses one of the three abilities of his Mangekyo Sharingan, though he tries to avoid doing so as it is taxing on his body and damages his eyesight.The first, Tsukuyomi (月読, Tsukuyomi), allows him to torture opponents for what seems like days in a matter of moments. Amaterasu (天照, Amaterasu), the second, generates black flames at his focal point that will continue to burn until the target is reduced to ash. The final ability, Susanoo (須佐能乎, Susanoo), creates a massive ethereal warrior that fights on Itachi's behalf and shields him from all attacks. Before he dies during his battle with Sasuke, Itachi implants his Amaterasu in Sasuke's eyes, hoping that doing so will keep him safe from Madara Uchiha.