Oleg Madrczyk was a doctor and torturer for The Covenant. He was the one who brainwashed Sydney and convinced her to work for The Covenant.
Season 3
Oleg was first seen inside of a torturing chamber holding a cleaver after a CIA agent named Peter Klein was kidnapped. He later decapitated Klein off-screen.
Oleg was seen again torturing Klein's partner when Sydney showed up and told Oleg to freeze. Oleg ignored her warning and took out a gun to try and kill her, but Sydney fired first, hitting him in the chest. In his last words, Oleg said that Sydney "was her favorite" because she never broke.
It was later confirmed that Oleg was a key operative for The Covenant and that he spent nearly half a year brainwashing Sydney, turning her into a unique member and assassin for The Covenant. However, before she was brainwashed, Sydney vowed that she would kill Oleg...which ironically, she did.