Leo Novokov was a Soviet assassin trained in the 'Red Room', who was placed in a state of suspended animation for decades. Awakened by mistake before he was due to be, Novokov became mentally unstable on learning that the USSR no longer existed and the war he had been trained to fight had never even begun. He began manufacturing his own 'Cold war' by instigating a campaign of murder and terrorism which brought him into conflict with the Winter Soldier, and brainwashed the Soldier's lover, the Black Widow into attacking her friends.
Later he used her to aid him in his plan to take revenge on Winter Soldier by initiating a terrorist attack on Washington. They both were at Arlington National Cementery near Captain America's and Bucky's monument when Bucky ambushed them both. He used Natasha as a human shield to gain upper hand against Bucky but luckily Hawkeye came to help by shooting Natasha into her foot and Bucky shot Leo in the head.
Leo was treated by S.H.I.E.L.D. and somehow survived the bullet going through his head. Natasha's mind was recovered back to her original state with her memories returning except for those with Bucky which have been wipen out permanently, much to Leo's delight as this is his final triumph against Bucky even though S.H.I.E.L.D. offered Bucky to find a way to recover these as well and he refused.