She does have a girlfriend! Her girlfriend is a doctor. (I'm not sure it's ever explicitly stated that she's a medical doctor, but fanon seems to like the idea that she's an ER doctor. Probably because it opens up chances for her to treat the main characters for the awful things ficcers do to them.)

You should watch more episodes. Diana is only in the first and last episodes of S1, but then she comes back and becomes a major character in S2 and following, and completely rocks my world. And it has been an enduring sadness that Diana and Elizabeth have next-to-no interaction onscreen.

Also, you should get on AIM. :P