Kabuto Yakushi

As a young boy, Kabuto was found amongst the dead enemies after the Battle of Kikyō Pass by a captain of the Konoha Medic Corps. The captain brought Kabuto to Konoha and raised him as a son. As no one knew of Kabuto's origins, Konoha decided to use him as a spy. He was taught Medical Ninjutsu so that the enemy would easily trust him. As a spy for Konoha, he traveled through many countries.

At some point in time, Kabuto was recruited by Sasori of Akatsuki, who sent Kabuto to spy on Orochimaru by usingMemory-Concealing Manipulative Sand Technique to transform him into a sleeper agent. Orochimaru discovered this, and released Sasori's control over Kabuto's memories, leaving Kabuto with the option to follow him. With a little persuasion, Kabuto agreed to side with Orochimaru.

Kabuto has since used his experience with spying for Orochimaru, playing the role of Sasori's servant to gain knowledge of Akatsuki's plans. Through his affiliation with Konoha, Kabuto was also able to acquire the rank of genin and enter the biannual Chūnin Exams, using the first two parts of the exam to gather information on his fellow participants for Orochimaru's use. Additionally, with the medical expertise taught to him by his adoptive parent, Kabuto is able to carry out human experiments for Orochimaru, while at the same time increasing his own knowledge of the human body.

SOURCE: http://naruto.wikia.com