It might be Marisa since she is known for using handguns, but looking at the type of gun it is (I'm pretty sure she uses revolvers in Kill Geppu, if my memory serves me correctly) I'm not entirely sure. It could be Remilia since she, too, uses dual handguns (see Udongein vs. Remilia). Or maybe--just maybe--it might be Satori!
I am conpuzzled about who it may be, but this might be a picture of Remilia here.
My money's on Marisa, on the grounds that she was wearing the same type of gloves in the previous picture. Also, Remilia did use dual 1911s, but only after she disarmed Reisen of them.