Zander/The Guardsman is a wraith (and the only one to beat the curse).
In the words of one fan:
It just all makes sense. His realization that "there is another one" at the end of Summerween Returns. The fact that he knows so much about paranormal stuff (like the wolfbane he cultivates). The fact that he doesn't know nothing about cooking, neither what people likes to eat. His family living in a farm? Doesn't fool me, for sure they've been dead for a good couple of centuries. How many sons of farmers end up being superstars? [...] I just wanted to retire in the craziest town in the world, for the pure air.
Then when Zander tries to get out of town in Broken Earth, he reveals that he knows Wendy is a wraith, and he has not been told that particular piece of information. Even later, Wendy herself starts to suspect him- that above all should be proof enough.
EZB is actually Alex Hirsch.
Yuki is a Time Lord.
Mabel mentions him maybe having two hearts, and after what happens to him in Fires of Hate, he reveals that his species can go into something that is best described as a 'repair-coma,' which one fan noted sounds like the regeneration abilities of Time Lords.
Alvis is going to betray Graupner.
He has made no attempt to hide his contempt for the Warlock (at least from fans' perspectives), and some fans believe it's only a matter of time until the truth comes out. The real question is will Alvis' betrayal help the twins or make things even worse.?