To call Hayley Atwell’s character a “destructive force of nature” is a bold description and it definitely indicates that she’ll be a major player in Mission: Impossible 7 and Mission: Impossible 8 rather than someone just passing through.
Christopher McQuarrie didn’t clarify whether or not Atwell’s character will be testing the protagonists as a fellow IMF member or as an antagonist, although he did note that Atwell has established a “vibration” with franchise lead Tom Cruise.