Background
At the end of the Third Shinobi World War, Shisui witnessed his best friend die, consequently awakening the Mangekyō Sharingan. A year later, he met a 5 year old Itachi Uchiha, becoming best friends and forming a brother-like bond. When Itachi began to act oddly, Shisui was instructed by the Konoha Military Police Force to monitor Itachi because the younger Uchiha's position in Anbu was instrumental to the Uchiha's future status in Konoha. Unbeknownst to the clan, Shisui in reality sympathised with Itachi, not wanting the Uchiha to go to war with the village. In the anime, Team Shisui was assigned a long mission, which they were tasked with chasing away Kirigakure forces being lead by Ao. The team completed their mission after Ao realised they were facing Shisui and had his team retreat, not without retaining an acute memory of Shisui's chakra colour. As the Uchiha continued to grow angrier and more violent from the distrust of the village due to the suspicion of their involvement in the Nine-Tailed Demon Fox's Attack, Shisui approached Hiruzen Sarutobi revealing the truth to the Third Hokage. Trusting Shisui, seeing as he is Kagami Uchiha's descendant, Hiruzen allowed the Uchiha to spy on his clan for the village. Shisui was perplexed that the clan would not deter from the coup d'état that they were planning and soon became greatly concerned with the consequences that would follow their actions. Ultimately, it was decided that Shisui would use his Kotoamatsukami on Fugaku Uchiha to subtly end the coup d'état plan.Later, as Shisui prepared to make his move, Danzō Shimura, believing that simply manipulating the Uchiha's leader wouldn't end the conflict and that he would eventually try to manipulate him as well, chose to take Shisui's eyes for himself in the desire to protect the village in his own way. While Shisui effortlessly subdued Danzō at first, the ruthless elder caught Shisui off guard by using the forbidden Izanagi with another Sharingan already in his possession to break free and steal Shisui's right eye. As Danzō and his men surrounded Shisui to take the left eye, Shisui managed to escape.Fearing that Danzō was right in Shisui's inability to stop the Uchiha's revolt and that the elder would continue to pursue his left eye as well, Shisui entrusted it to Itachi, telling him to protect both the village and the Uchiha name. Shisui wrote a suicide note telling his clan he cannot follow through with the coup d'etat, but his clan's narrow mindedness made them unable to understand this, believing he would even sacrifice innocent lives for the sake of the Uchiha Clan. The contents of the suicide note also made it appear as though he had crushed his eyes when jumping off a cliff into the Naka River to kill himself in order to prevent conflict from arising over his eyes within the clan. He, at the same time, was able to erase his existence, leaving no corpse behind. In the anime, Shisui was also hoping to use his death to awaken Itachi's Mangekyō Sharingan, succeeding in the goal.

Personality
Shisui was a very humble and down-to-earth individual. He was never arrogant about his own talent or accomplishments, yet was not above telling others when they made mistakes. At the same time, he was open-minded, never stubbornly believing in one ideal solely and strove to see that unique nature of current situation before him. Shisui was also a deeply compassionate man, even to enemies, showing sympathy and kindness to a dying Mukai Kohinata despite having been ordered to assassinate him as a traitor. He was also shown to be very perceptive and intuitive even from a young age, able to see through the emotional nature of others and likewise able to notice the smallest details around him. Shisui came to care for Itachi very much; in fact, he was like a big brother to Itachi. Shisui taught Itachi that "self-sacrifice… a nameless shinobi who protects peace from within its shadow…" was the true mark of a shinobi. 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. What is really important to know is that, like Itachi, Shisui doesn't appear to enjoy killing people and prefers to use non-lethal methods whenever possible as shown in his using a genjutsu to scare off Ao's squad when it was stated he could have killed his entire squad with ease. Shisui also used a genjutsu on Danzō, which would disappear shortly, instead of killing him when the latter tried to steal his eyes.

Abilities
Even by Uchiha standards, Shisui was noted to be one of the most talented members the famed clan ever had. Danzō even hailed him as the strongest Uchiha of his time. His abilities allowed him to rise to rank of jōnin at a young age and eventually a captain of his own squad. His prowess was so widely known that an entire squad of Kiri-nin quickly chose to flee upon learning of his presence rather than to confront him. A further testament to his prowess is the fact that Kabuto Yakushi wanted to reincarnate him for the upcoming war but was unable to find a sufficient DNA sample for it.

Physical Prowess
He had earned the nickname of "Shisui of the Body Flicker" as he had great mastery of the Body Flicker Technique. Shisui would often make practical uses of the Body Flicker Technique to fade out of sight, leaving no trace of either his presence or indication of his destination. In the anime, his velocity and mobility was so great that he could leave behind multiple tangible after-images that could strike with actual force and could also move undetected by sensor ninja. In battle, his speed was shown to be able to seamlessly dodge barrages of projectiles from his enemy and likewise outpace them. He was also a proficient taijutsu user, being able to defeat Itachi in several sparring matches. Furthermore, he displayed a high level of pain tolerance as he didn't flinch from having either of his eyes removed.

Ninjutsu
Shisui was able to summon a flock of crows to aid him in battle and master three basic nature transformations (Fire, Lightning and Wind) along with the Yin Release. He was very skilled with using fire techniques such as the rite of passage Great Fireball Technique. Shisui's usage of this technique required the efforts of many Anbu-level Water Release users to counterbalance it. He also wielded a tantō, showing considerable skill in kenjutsu from a young age against two Anbu. He showed similar remarkable Shurikenjutsu-prowess to Itachi and likewise demonstrated noticeable sleight-of-hand with multiple kunai. Shisui also proved to be a very skilled tracker.

Mangekyō Sharingan
Shisui was one of the few members in the clan's history that was able to awaken the Mangekyō Sharingan.He awakened it when he watched a dear friend and rival die after withholding his aid in a moment of envy and weakness. Its design is a four-point pinwheel and it granted him access to a powerful genjutsu, a unique technique noted to be of the highest calibre, Kotoamatsukami.his technique allowed him to enter a person's mind and manipulate them by giving them false experiences, making it seem as if they were doing things on their own free will; the victim wouldn't even notice they were being manipulated. In the anime, this technique was so powerful, Shisui believed he could use it to make the entire Uchiha clan abandon the coup d'etat. Shisui's Mangekyō Sharingan also awakened the ability to manifest Susanoo. It is green in colour, when manifested had a fiendish look to it. Its defence was capable of withstanding explosive tags attached to kunai, leaving Shisui untouched. His Susanoo demonstrated the ability to unleash a barrage of chakra needles from its chest at an incredible speed.

Source: http://naruto.wikia.com/wiki/Shisui_Uchiha