Danzō Shimura

An elder ninja of Konoha, is a war monger whose politics operated in opposition to the Third Hokage's politics. Within the ANBU corps, he created and led the training division known as Root, which was detached from the main ANBU. The Root group was disbanded and he lost his position, however many of its old members are still loyal to him. He is still a major piece in the power play of Konoha, though with his own influence within the village. He is currently targeted for assassination by Sasuke Uchiha, along with the two Kohona elders, Homura Mitokado and Koharu Utatane. It was later revealed (when he was talking to ANBU root) that his secret plan is to overthrow Tsunade and become the Hokage.

Background
In the past, he competed with Hiruzen for the position of Third Hokage. He created the (now disbanded) ANBU training department known as and acted as its head. Danzo was the one who recommended that Sai be placed in Team Kakashi. He appears to have lost an arm at some point, the cause of which remains unknown, and may have also lost his right eye.

Even though he is not an official part in the leadership of Konoha, the two Konoha Elders still hold him in high regard and have often taken his advice when dealing with the politics of the village.

He later on took part in the dealings with the Uchiha clan, when they learned that they were planning a take over of the village. He took part in planting Itachi Uchiha as a spy in his own clan, and it is assumed that he ordered the annihilation of the clan in order to avoid a civil war that would lead to the destruction of the village.

However after assignment was over, it is somehow revealed to him that Itachi was unable to kill his little brother, and he was later threatened by Itachi, that he would tell everything if he ever touched Sasuke.

Personality
Danzo is a blunt tactician, which is demonstrated by his preference of eliminating threats and ordering them executed instead of using negotiation. His ideology regarding shinobi is parallel to that of fascism.

Although not an official part in the affairs of Konoha, he still maintain an eye on everything, and will immediately act if he sees anything that might look like a threat to Konoha, so much so Itachi felt the need to threaten him not to harm Sasuke after the Uchiha Massacre (considering Sasuke was seven years old at the time and wasn't aware of the story behind the massacre, he didn't represent any viable threat).

Even though he addresses Tsunade politely, he is not pleased with her. This is mainly because she is the granddaughter of Hashirama Senju, the First Hokage, and like Hiruzen, has inherited the same teachings and philosophy as him, something that Danzo opposes.

Sai and Sasuke arc
Despite Danzo's apparent support for the Hokage and Konoha, Tsunade does not trust him; this is apparently well founded, as Danzo had given Sai a secret mission that Tsunade was unaware of: sending Sai to contact Orochimaru and offer to act as a subordinate for him. Yamato believes that Danzo is trying to take over Konoha with Orochimaru's help. This is apparently proven when it is revealed that the envelope Danzo sent Orochimaru was full of pictures and information on the true identities of Konoha's ANBU. However, all of these are nothing more than a front to let Sai get close to Orochimaru, so that Sai can execute his true mission: the assassination of Sasuke. After learning of the failed assassination attempt of Sasuke, and Sai's attachment to Team Kakashi, he merely states that "emotion brings hatred, and hatred brings war."

Fire Temple Filler arc
He is later seen followed by ANBU agents, on Tsunade's orders, when he is to be briefed by a spy of his, from Amegakure. He is then aprehended alongside his spy, by the ANBU agents and is questioned by Tsunade, due to his late mysterious actions. However, the interrogation is later interrupted by Sora, who was found eavesdropping.

Pain's Invasion arc
When Tsunade and the others leave her office, the toad that Tsunade made responsible for calling Naruto back from his Sage Training is killed by Danzo, using his sword. He then mentions that the Nine-Tailed Demon Fox is now out of his way, perhaps referring to the little faith he has in Naruto and a desire to keep the Nine-Tails out of Akatsuki's hands, or for a desire to see Konoha destroyed by removing the possible help that Naruto could give. Danzo reveals to his subordinates that his plan is to become Hokage. How he plans on doing this when he himself cannot use ninjutsu is unknown.

Past Speculations
Fans have speculated that Danzo could also be Madara Uchiha, Tobi has bolts on the same arm that Danzo has covered up, and similar hair styles. Also, Danzo and Madara have the same three scars on the right cheek, and in the fire temple arc before he was interrogated, Danzo was meeting a spy from Amegakure.