Shisui Uchiha

was feared as, and was one of the most talented of the Uchiha clan.

Background
At some point in the past, Shisui fought with Ao. Ao retains an acute memory of Shisui's chakra and ability to control others with his Sharingan. Because of this, he recognizes that Shisui's right arm and eye were transplanted into Danzō.

About six months after joining the ANBU, Itachi began to act oddly, and Shisui was instructed by the Konoha Military Police Force to keep tabs on him. Soon afterwards Shisui was found drowned in Nakano river with a suicide note stating that he was tired of carrying out the clan's duties. The police suspected Itachi had murdered him and used his Sharingan to fake the suicide note, since Shisui was very devoted to the clan, and suicide was the least likely thing Shisui would commit. After this, Danzō took Shisui's right arm and Sharingan and implanted them into himself. Itachi later confessed to Sasuke that he did kill Shisui and forged the suicide note, all in order to awaken his own Mangekyō Sharingan.

Personality
Shisui was Itachi's best friend; in fact, Itachi thought of him as an older brother. However, Shisui was also very devoted to the clan, which is why the Konoha Military Police Force asked him to spy on Itachi. It is because of this devotion that the police did not believe he could have committed suicide.

Abilities
Shisui was noted to be one of the most talented members of the battle-excelling Uchiha clan. He had earned the nickname of "Shisui of the Body Flicker", possibly implying that he had great mastery of the Body Flicker Technique. As a wielder of the Sharingan, Shisui possessed an apparently unique dōjutsu of the highest caliber. It allowed the user to control the mind of the target in the most discreet manner.

Shishui also had the ability to change gender on command.

Trivia

 * Shisui's name has two interpretable meanings. Combining the words and, Shisui would mean "death water," which is a reference to his death by drowning.  alone means "'still water'" or "'stagnant water'."