At the subway when Mercy and Fox were running down the stairs from the cops, they showed a back shot of Mercy and it was clearly another woman. Her hair was about shoulder length and straight. Mercy's hair is longer, and curly.
Originally, Fox was supposed to be Mercy's love interest, but the two actors had no chemistry. This resulted in the love interest being changed to Swan, which caused the actor who played Fox to leave film. His death was written in to deal with this.