Lisa Chambers in "Double Shock" - "the lovely but not too brilliant young lady" who falls for the lawyer's trick and then falls to her death.
Marcia Dalton in "A Stitch in Crime" - Sharon Martin's roommate who is "inner directed", cures allergies with sips of water and takes quick peeks in mirrors during murder investigations.
Gloria West in "Greenhouse Jungle" - "friend" of Tony who thinks she's going to outfox both Jarvis Goodwin and Lt. Columbo
Anyone who has a problem with "New Age" type people (I don't) might find Lisa Chambers at least a little unstable just because of that.
Arlene Martel causes me to have such a thing for Gloria West that I can't be very objective about her. Though she does seem pretty vindictive toward Cathy (even if she does believe she's guilty of the murder).