When one use a soft filter, the entire picture is made a little "hazy" with less harsh lines, which ladies may prefer, if I am allow to make cliches statements. However such softness may not be desirable when photography fishermen with all the "wonderful" lines.
Regarding "softness" in lenses used wide open - the image should be sharp where it should be, and out of focus when it is desired to be soft or out of focus. For example, in using a 80mm f1.4 lens wide open, one should be able to count all the eyelashes, yet the ears out of focus, giving an illusion of depth in a two-demsional image. This image is technically not "soft", but parts are rendered out of focus for esthetic reasons.
When a lens is called "soft" we may be talking about a poor quality lens which do not focus well, resulting in nothing in focus.
Coming back to the original question - what filters for portrait. The question I will ask is this. Are you making black & white portraits, or colored. Film or digital. Without more information, any answers will not be properly directed.
One more filter, ND filter if you're thinking of shooting wide open in bright daylight and it goes beyond what your cam's shutter speed can cope... e.g. if you want to shoot f1.2, ISO 100 in broaddaylight, you'll need shutter speed of 1/6400s.