Shisui Uchiha was feared as "Shisui of the Body Flicker" (瞬身のシスイ, Shunshin no Shisui) 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 Mangekyou Sharingan, forging Shisui's handwriting on his suicide note with the help of the Sharingan. Shisui acquired the Mangyekou Sharingan and a technique called Kotoamatsukami.

Shisui wanted to stop the Uchiha's Coup d'Etat against Konohagakure with his Kotoamatsukami, but Danzo stole one of his eyes and so he gave his other eye to Itachi to protect the village and jumped off a cliff and died, then later found on the river by the Uchiha military policemen. His eye helped Itachi to free him self from Kabuto's Reanimation control.

(Source: Narutopedia)