Aku

Aku is the main antagonist to Poji. He is the scientist responsible for the Anger Cells. He was exiled by Pain and Konan for creating such thing. Aku went into hiding the and established the Black Ninjas.

Background
Aku was a original going to be a test subject for Orochimaru but decided to leave to pursue his goals to create a new Tailed Beast. He return to the Land of Rain and worked on the Anger Cells creating them from the Cells of the Nine-Tails and using the power of the moon to further his plans. He then kidnapped several children to test his experiment but only one is a success but before he could do anymore Poji transformed into a half wolf monster and killed his parents destroying the lab. No one but Poji, Aku, Kajin and Karuto survived. Aku then was exiled from the Land of Rain by Pain and Konan and journeyed to a new lands where he established the Land of Black Ninja and the Black Ninjas.

Eternal Mangekyo Sharingan
Aku stole Madara's Eternal Mangekyo Sharingan while he was still active. He didn't gain the full extent of Madara's powers but he's jutsu were greatly strengthened and able to copy jutsu.

Rinnegan
Aku also stole the Rinnegan from Tobi strengthening his abilities even more. Allowing him to uses the Rinnegan abilities as well as able to control a Meteor just like Madara.