Hi kiksumazz,
The best Pentax lens for portrait photography is the new premium Pentax portrait lens -- the DA* 55mm f/1.4.
http://www.pentaximaging.com/camera-lenses/smc_PENTAX_DA_Star_55mm_F1.4_SDM/
http://www.bhphotovideo.com/c/product/594340-USA/Pentax_21790_55mm_f_1_4_DA_SDM.html
The DA* 55mm f/1.4 replaces the old 85mm f/1.4 portrait lens. 55mm on Pentax digital cameras is about the same field of view as 85mm on film. The focal length multiplier for the K20D is 1.5385. 55mm x 1.5385 = 84.6mm, and focal lengths on most lenses are rounded.
The FA 50mm f/1.4 is also a fine choice for portraits, and I use it for that. The bokeh (blur of out-of-focus areas) is beautiful, though probably not up to the 55/1.4.
The new "II" version of the 18-55 kit lens is generally excellent for a kit lens and a great value, and not a bad lens to have for general shooting when you want to travel light. But it won't allow throwing the background as much out of focus (f/5.6 at 55mm compared to f/1.4), and it's bokeh won't be as pretty.
If you want a medium zoom to double as a portrait lens, I would look at:
-- Pentax DA* 16-50mm f/2.8
-- Tamron 17-50mm f/2.8
-- Sigma 18-50mm f/2.8
-- Sigma 24-70mm f/2.8
-- Sigma 24-60mm f/2.8
However, don't expect the best bokeh from any such zoom. I have the highly regarded Tamron 17-50/2.8, and I like it very much for a medium take-everywhere zoom, though I had to send it to Tamron for warranty service (it had a bad case of front-focus). But on difficult bokeh such as trees and bushes, it doesn't look as pretty as the 50/1.4. It's fine with easier bokeh such as large, rounded shapes.
Good luck and have fun!
Greg