IMHO the Nikkor 50mm f1.8 is sufficiently good for portraits. Unless you dun mind paying much more for lenses with better sharpness, excellent bokeh and colours. Why not shoot a little more and enjoy the process before deciding on getting more lenses?
The Tamron 17-50mm f2.8 suggested earlier on is quite good. Good value for money
Any lens is a portrait lens. Wide angle, standard, medium telephoto or super telephoto. All are lenses you use to capture portraits and all perform as they are made. It is you that make the portrait look nice. The lens is only in charge of allowing light to fall onto your camera sensor.