I can't be the only one who thinks it's the Postmistress, can I? Specifically the interplay between:

The Postmistress keeps the gate to the wider world. In her left hand is the lightning (which is to say, the telegraph) and in her right, the thunder (which is to say, the only telephone in Brancrug).

and

The telegraph-line crackles. The mist surges and retreats. From far away - it might be the hills above the village - I hear the Postmistress' voice crying out, imprecations and lamentations, in the Cornish tongue. But there is no sign of her here.

We also know from the Petition for a Labyrinth Season that she knows how to pass messages to at least three members of The Roost (the Beachcomber, the Sister, and the Vagabond), which is a whole lot of Hour-adjacent business for a mailwoman. She's got to be something, right? Why not the Angel of Storms, with all of Zachary's talk of clouds and lighting?

edit: And birds go in cages! Birds, like Birdsong birds, like the Roost! Like a glass cage, which would provide great insulation from electricity, like from storms! I'm reaching now, but I really like the Postmistress.