I really like #5 . . . nice mood, nice light. A smidgen of fill would not hurt, but not really necessary. I'd say this one is a keeper!
On images 1, 2, and 4 the first thing I would suggest is to turn the subjects head more towards the camera. The nose breaking the line of the cheek is considered a no-no.
On images 1-4 the shadows and harsh spectral high lights are a result of the flash being direct, harsh and on-camera. If you don't have a flash cord or remote triggers to get the flash off-camera, then you can swivel it to bounce from a neutral colored wall/ceiling and move the subject so that the light is bouncing back at about a 45 degree angle. If you don't have a neutral colored wall, then it can be reflected from a white sheet, drapes or even a large newspaper taped to a wall. You can even bounce it from an assistant wearing a white shirt . . . anything that will not create a color cast.
There are even some flash modifiers that can give you the effect of an off camera flash, like the Peri-Scoop.
http://youtu.be/JCPhl5jd4rEWith the goal of taking better portraits, I would use the lens that you have for now making sure that it is zoomed out to the full 55mm for the most flattering perspective, and spend a little on some lighting equipment, like a set of remote triggers and an inexpensive, shoot through umbrella. Total cost should be less than $70.00.
Here is an example of a portrait taken with the flash pointing at a far corner of the room where the wall meets the ceiling. (Flash bounced up at a ceiling does not produce nice portrait lighting)
Hope that helps.
I really like #5 . . . nice mood, nice light. A sm... (