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. However, when Itachi began to act oddly, Shisui was instructed by the Konoha Military Police to keep tabs on him. Soon afterwards Shisui was found drowned in Nakano river. Although it appeared to be a suicide, the police suspected Itachi had murdered him and used his Sharingan to fake a suicide note. Although, Itachi later confirmed this, the circumstances of his death are still a mystery.

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.

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. "Shisui" alone means "'still water'" or "'stagnant water'".

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