So where's everyone at after the first season? They're not a family yet. Twilight's had the most development by far with Anya coming in second. Yor's almost a joke character by comparison. *Technically* Yor is expendable as far as the mission goes. He needed a wife to get his foot into the door at the school and while keeping up appearances is important, calling her vital would be an exaggeration. Twilight seemed to imply that keeping Yor on his wife would help deal with the secret police somehow, kind of hiding in plain site, that sort of thing, but that makes her an asset or a tool.
Even so, originally Twilight seemed to think she only had to be kept on as far as the interview stage of enrollment. And Yor thought that was as far as things would go as well, but he flubbed things when he met her coworkers, and they just kind of went along with it.
We almost know everything we need to know about Yor. I don't think she's abandoned her personal happiness in the same way as Twilight has. She just doesn't seem particularly interested in getting into relationships. She's only in one because she doesn't want to draw the wrong attention.
She likes killing, but maybe not? There's a bit of a thread there with the Lady Macbeth allusions hinted at, but despite having a telepath in the show that gives access to everyone's musings, Yor's internal thoughts are a source of humor. Yor thinks about killing/death/etc and Anya reacts.
So as things stand, Yor's overall proven to be very useful, is low maintenance, and not much of a liability. She brings a great deal as far as audience enjoyment, since she's a source of laughs, action, and is nice to look at...but I don't think as much thought and effort is put into her character as the others.