Shisui Uchiha

was feared as and was one of the most talented of the Uchiha clan. Shisui was the best friend of Itachi Uchiha; in fact, Itachi thought of him as an older brother. He was found drowned in a river from what appeared to be suicide, though the Konoha Military Police believed it to be murder and the police asked itachi did he do it and then he went besurk on the police and when sasuke looked at itachi for a second he showed sasuke his mangekyo sharingan. It was later revealed that Shisui had been ordered to keep tabs on Itachi. Itachi later admitted to Sasuke Uchiha that he had murdered Shisui to obtain the Mangekyo Sharingan, forging Shisui's handwriting on his suicide note with the help of the Sharingan.

Trivia

 * Combining the words Shi (Death) and Sui (Water), Shisui would mean "Death water", which is a reference to his death by drowning.