Kabuto Yakushi

Orochimaru's former right hand man. Now one of the major villains in the story. Helping Tobi create an army against the shinobi alliance.
Kabuto's life is steeped in Mystery. The first Mention of him, he was found wounded at the batttle of Kikyo pass. He was taken in by a Konoha Medical-nin, who taught him everything he knew. Kakashi's first statement when he sees Kabuto in the hospital with an unconscious Sasuke at scalpel point was "Oh, you're that medic's son" , making Kakashi seem more knowledgeable about Kabuto than he had previously shown.