Henry Cavill played August Walker, who is revealed to be John Lark, in Mission: Impossible – Fallout.
Walker is assigned to accompany Ethan Hunt on the mission to recover stolen plutonium. The film gradually links him to the extremist network known as the Apostles and reveals that his public identity is a cover for the mysterious terrorist John Lark.
This reveal is central to Fallout’s plot because Ethan’s team is trying to prevent the Apostles from carrying out a nuclear attack. Walker’s CIA credentials allow him to move close to the operation while hiding his true allegiance.
The answer is the actor’s name, not the character’s alias. Henry Cavill plays Walker, while Sean Harris plays Solomon Lane and Henry Czerny returns as Eugene Kittridge.