Good advice already. Nothing I'm saying counters it. I offer this just as another way to understand the issues. My apologies if the explanation is too basic, but I don't want to skip anything important.
Normally, with mixed light, we think about getting ambient where we want it first (and for good reason). Once you have the background as light or dark as you want it, you note your exposure. From that point, you can set your strobe to match the aperture and see how you like the image. This gives you a "proper" exposure on your subject. You can then dial the flash down a bit (between a partial stop and two stops, depending on taste) if you want more of a fill light ("natural") look.
The image you showed has the exposure set lower than the background, which is fairly dark. If you measure the rim light hitting her shoulders, that would be higher, of course. Rim light should generally be as high or higher than the exposure. Her face, like much of the background, will be shadowed. Which is where the flash comes in.
If your flash is under the exposure value you have set for your camera, this is fill flash. This will give a more "natural" look. When you get the flash up to the exposure (ISO and aperture) set on the camera, it is no longer a fill. Instead, it becomes your key light.
This image looks to have flash set about as high as the exposure set on camera. Which explains why DrakeJ is saying it doesn't look "Natural" --the exposure is coming from the flash in front, not the environment as it stands. I hope this also sheds some light on what nemophoto is saying.