This discussion may contain spoilers!
So I've recently discovered this and Im not sure how many other people are aware of this, but I have a good guess at who he actually is. While we hear a few mentions of him being the one who the tweedles are working for, the identity is unknown, but we know it is referring to the man from the nursery rhyme, which mentions the crooked man with the crooked cat that caught a crooked mouse.
It's noted in the book of fables under the profile of bluebeard, that "Even if he was able to leave his violent ways in the Homelands, that hasn't stopped him from making the occasional trip down CROOKED LANE." with that information we can guess that he (the crooked man) is likely the antagonist of the series, ever since the suggestion that he would likely be taking over after crane, because of the scene at the "next time" segment, shown at the end of episode 2. Makes sense cause he likes decapitating women. So no-brainer.