There are a lot of variables here, some depending on the scene and some just depending on what you want. Either way, though, this is certainly a lot more about the scene and the lighting than about any specific camera or flash settings. There's no magic numbers which will make this work.
You can light your subject entirely with candlelight, but usually that requires a long exposure and accepting some blur, probably in combination with a rather high ISO and the associated noise — or else a really big candle. Since the pool in the background is lit (even if dimly), supplemental lighting for your subject with flash seems like a good idea.
You might be able to do with a bare flash on a lightstand, but better, use a softbox — big as you can easily transport and just out of the frame. (Or an umbrella — more portable, more awkward to use.) It's easiest to work with two such lights — perhaps one to each side and a little forward of the subject — but you can also do amazing things with just one light. (The Strobist 101 guide is an excellent starting point.)
As far as settings: start with figuring out your exposure parameters for the background. You'll want a shutter speed somewhat slower than the flash's sync speed — it may help to have a tripod or image stabilization. Then pick an aperture that gets you the depth of field you want, and finally set ISO at a level that gives you the overall brightness you're aiming for for the background and surrounding scene. (Since you mention "low-key", presumably you want that fairly dark.)
Once you've got that, add in the flash for your subject. It's a good idea to get this set up beforehand with a stand-in if you can, unless your model is extra tolerant. Put your flash in manual power mode. Start at very low power and see what that does; increase until you've got what you want.