Shisui Uchiha

was a shinobi from Konohagakure's Uchiha clan and was renowned as.

Background
Shisui was noted to be one of the most talented members of the Uchiha clan. At some point in the past, he fought with Ao. Ao retains an acute memory of Shisui's chakra and ability to control others with his Sharingan.

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 the Naka River with a suicide note stating that he was tired of carrying out the clan's duties. The police suspected that Itachi had murdered him and used his Sharingan to fake the suicide note, since Shisui was very devoted to the clan, and suicide would have been highly unusual and uncharacteristic. Itachi later told Sasuke that he did kill Shisui and forged the suicide note, all in order to awaken his own Mangekyō Sharingan. Shisui's corpse has apparently disappeared, as Kabuto Yakushi cannot locate it.

However, the truth is that Danzō Shimura had taken one of Shisui's eyes while he was still alive. Shisui then entrusted the other to Itachi telling him to protect the village and in order to prevent conflict from arising over his eyes, made it appear as though he had crushed them before dying. He would later erase his existence leaving no corpse behind.

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.

Along with his clan, Shisui was devoted to the protection of the village which he cared for deeply. With this in mind, he entrusted his left eye to Itachi and told him to protect the village. According to Itachi, he and Naruto Uzumaki held similar ideals and goals.

Appearance
Shisui had dark unkempt short hair and a flat nose. His eyes were also presumable dark as customary of most of the members of the clan (although his eyes have not been seen without the Sharingan activated). He also had markings around his eyes that were turned upwards at each end. In the anime however these lines are broken instead of whole. He seemingly also wore the standard attire of the Uchiha clan which includes a high collared dark outfit along with a tantō strapped to the right side of his shoulder.

Abilities
Shisui was noted to be one of the most talented members of his battle-excelling clan. He had earned the nickname of "Shisui of the Body Flicker", possibly implying that he had great mastery of the Body Flicker Technique. He was hailed as one of the most powerful genjutsu users of the Uchiha clan. Another testament to his prowess is that Kabuto Yakushi wanted to revive him for the war but failed to find sufficient DNA samples.

Sharingan
Like many members of his clan, Shisui was a wielder of the Sharingan. He was praised alongside Itachi for having the greatest mastery of and sheer power in its usage.

Mangekyō Sharingan
Shisui possessed the Mangekyō Sharingan. With his Mangekyō Sharingan he could use a unique genjutsu of the highest calibre: the Kotoamatsukami, that allowed the user to control the mind of the target to the point where even the one it was being used on wasn't aware of it themselves. This ability was apparently so unique and powerful, that Madara Uchiha himself (despite having access to both the Eternal Mangekyō Sharingan and the Rinnegan) coveted it, and was angry that Danzō managed to destroy the eye that was transplanted into him from Shisui at the moment of his death; Kabuto was extremely thrilled when he learned that the remaining eye is within Itachi's crow. The genjutsu could only be used once a decade, unless the user has Hashirama Senju's cells, by which the recharging time can be drastically reduced.

Legacy
Throughout his lifetime he influenced Itachi greatly as a friend, teaching him that self-sacrifice and that the true ninja was the one whom protected the village from within its own shadow, he laid down the foundation for his future. Even years after his demise, Shisui's powers and actions mark the world. Both his Sharingan have been used by Danzō and Itachi to protect Konoha in their own ways.

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".