There is no such thing as a portrait lens. Just some are known to produce more appealing portraits. It can be used to shoot any other thing where the focal-length desired is the same. Note that I didn't say needed since portraits can and are shot with different focal-length.

A 42.5mm lens on a MFT gives what people consider a flattering perspective, making people's face's proportion and nose length shorter than perceived with the human eye. Your 35-100mm lens, set to 42.5mm will give exactly the same perspective.

With an F/1.7 aperture though, which your 35-100mm lens cannot manage, produces an image with a more shallow depth-of-field. This is considered desirable for portraits since the background gets more blurred and isolates the subject more than an F/2.8 lens.

The bottom line is that you can make portraits which are flattering with your 35-100mm, but a 42.5mm lens could be used to isolate your subject more from the background, given the same situation. It will not necessarily mean your portraits will be automatically better since there are so many factors that influence the results such as subject, lighting, position, distance, etc.

Specifically regarding those two lenses, when it comes to the quality of the resulting images, there is a small difference between the two. The 42.5mm is an extremely sharp lens and shows very little corner shading (aka vignetting) while the 35-100mm is quite sharp - just not as sharp - and shows moderate vignetting until stopped down to F/4. Such difference in sharpness would be visible in large prints only. Although the vignetting is easily noticeable, it is one of the easiest artifact to eliminate in software.