Henry Cavill as August Walker, a CIA assassin working for the Special Activities Division tasked with monitoring Ethan and his team. He is also the real John Lark who contacted the Apostles to steal the plutonium cores.
Per Wikipedia under the Cast section.
Erica Sloan did not intend for Walker/Lark to take out Ethan's team and escape. It wasn't his team rather than her's/the CIA's. She didn't actually know that Walker/Lark had men amongst her team. Granted, it's not entirely clear how Lark managed to do this, but also not impossible for him to have his own men inside the CIA or just replacing Sloan's ...
Lark, in the context of this universe, is supposed to be a terrifying figure of anarchy/change (see his manifesto), who also works with Solomon Lane, who heads up the crime organization The Syndicate.
Keep in mind also, that the White Widow describes that her clients require the extraction of a political prisoner as the price for the two remaining cores. ...
As pointed out by eduffy in comments, this character is decoy Lark. Medium.com
also, have nice details about the character and also provide the information that he is played by stunt actor Liang Yang .