Dr. Faye Miller
There are two theories about Dr. Faye. The first is that, like any other woman who gets the hump-and-dump treatment from Don Draper, she sniffled a bit and then went away quietly, refusing to stoop to his level by making trouble. The second is that Dr. Faye has spent three years plotting her revenge, and like Megan's spaghetti, it's going to be a dish best served cold.
I'm down with the second theory. Are we supposed to forget how she once mentioned over dinner that her dad is in the mob? No way is a veteran of The Sopranos like Matthew Weiner tossing that cannoli out there without following up on it.
So even though Dr. Faye hasn't been seen (or even mentioned) since the end of Season Four (in "Tomorrowland"), I'm braced for her to come back and collect her pound of Don-flesh. We already know, as does Don, from overhearing her in the phone booth, she does not handle rejection well. She also knows his dangerous secret. But most of all, she has that mobster dad. So don't be surprised if the season ends in 1970 with Don dangling from a meathook to the piano solo from "Layla."