I love this kind of soft sidelighting (looks like window light). And it's okay to have it a little dark, but you need some fill on the opposite side, to avoid areas of complete black. You can easily get that with a white reflector -- something as simple as 16x20 inch white matte board, or piece of white foam core. This will kick some light back, giving you a little detail on the left side. If you can, shoot this again, in the same spot and same time of day, but with the reflector. Otherwise, very nice portrait. And I think you've got a great subject here who has the ability to be very natural in front of a camera. Would love to see a "serious portrait" of her, as well.