That's Lucy, a... Sort of friend of Rhoda's? They really don't get along. It's more like Rhoda and Lucy are both friends of Diane, and at LEAST one of them has a thing for Diane, so... tension.
As for Lucy's companion... I dunno his name, but I assume Lucy gave him that collar (after asking him to buy it for her) and somehow convinced him to wear it for her amusement, which certainly sounds like her.
As for Lucy's figure in the third panel, I wanted to try and have it look like most of Lucy shrunk, but leaving part of her the same size as before. I think, in retrospect, I would have preferred it if I had her evenly shrink, but I reached that conclusion at the inking stage while working on an already late comic, and I'm sure some people will like the results in any case.