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.
Raechael: Thought I'd give this a quick spin in PhotoShop. I brought the levels up, to add a little detail on the left side. But then the t-shirt was too bright, so I selected most of it and dialed it back. But it was still too obvious, so I added some blur, then erased the face and hair. Then cropped it tighter. Don't know if you'll care for any of this, but maybe it'll give you an idea or two. I actually like the mood of your dark original, just use a small reflector to show minimal detail in the hair on the left side, and a flag or scrim on the window (or key light) to knock down the light on the tee. In any case, in this cropped version, you can see the power in your original portrait. A very successful collaboration between you and Sara. Nice!
