There's a lot more here than simply an under-exposed subject. This isn't so much a portrait as an out-of-focus landscape shot with a person in the middle. The first "rule" of portraiture is that the image should be of the person, with minimal background distractions.
The first thing I would suggest is to consider the portrait (vertical) aspect when shooting a portrait. The next is move in on the subject. Way, way in. A little background is nice if it adds to the scene, but too much and it is no longer a portrait. Then, we move on to lighting. By the look of the light/shadow here you picked about the worst time of the day to shoot. No big deal, sometimes you have to shoot in adverse conditions. There are two choices; either spot-meter the subject for a correct exposure and blow out the background, or meter for the background and add fill-light to correctly expose the subject. Of the two, the latter is generally the better way to go.
My preferred method here would be to use a single speedlight on manual, with a diffuser, high and about 15-20 deg camera right. A large reflector 45 deg. subject left would have worked as well.
When composing, avoid things like the cropped wrist. Either a bit above or below (best choice) would be better.
Just my $00.02 worth - your mileage may vary.
~John