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. 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

 * Shisui's name has two interpretable meanings. Combining the words Shi (Death) and Sui (Water), Shisui would mean "Death water", which is a reference to his death by drowning. "Shisui" alone means "'Still water'" or "'Stagnant water'".