Well, I’ll try from what I’ve played so far, but I don’t want to spoil anything major, as some of the revelations come as climaxes or surprises along certain storylines. You can learn a lot from just the texts on item conversions, though.
In any case, for starters:
Tomb-Colonies: Not a united body, per se. They’re essentially those who ought to be dead, but aren’t. They range from the crusty and melancholic, to the "I’m dead anyway, so what do I have to be scared of?" Of the latter is Lettice the Mercy; she’s fun! They have a connection to the past cities, naturally; particularly the third (With Feducci specifically, though I don’t know much beyond that).
The Gracious Widow: Runs a more-or-less Yakuza organization, in competition to the Cheery Man and the Dockers. Strong connections to the Fourth city, and she’s rumored to have lived for much longer than you’d expect…
The Great Game: Spies spying on spies. They work for every sort of master (and Master), but their agendas more or less cancel out in the long run. That said, they work as more or less an essential flywheel to maintain the balance of power. The Game must spin on… and there’s good money in it, if you’re the clever sort.
Criminals: What you’d expect. If they have a leader, its the Cheery Man. Out for profit, and nuts to the law!
Society: The great and good, in their own estimation, at least. At any rate, the rich and famous. It is surprisingly easy to court their favor, once you’ve got your foot in the Shuttered Palace door. They claim to practice respectability, but there are a lot of secrets behind closed Townhouse doors, and as for the Empress herself? Well, there’s a tragedy behind that, as were all the Deals for the Cities.
The Duchess: A very independent Woman, and lover of cats. Somewhat feline in her own habits; and a strong connection to the second city. Rumored to be older than even the Widow…
Urchins: There’s more to them than you’d expect. The ignored often know what everyone else does not. The Gangs may be older than London; their members are certainly wise to the secrets of the Neath. And graduates from the Gangs (if they survive) turn up in some very unexpected places.
Dockers: The rough and ready; zailors and the like. But one also has here the Engineers, the bright mechanicians building the future through science! Rare to come across such geniuses, but if you want to cross the Zee, you’ll need brains as well as brawn…
The Church: The Anglican Bishopric, now headquartered in the Neath. Mostly wedded to routine, though the Scriptures have been through several revisions for Neathy relevance by this point. Most of the clergy may be considered more scholars than pastors, but one doesn’t see them do too much… Except for the Bishops of Southwark and St. Fiacres. The first is a crazy awesome dude who literally wrestles demons, and the latter is a shifty, but ultimately fanatical, searcher for true immortality…
Hell: Interested in only one thing. No one is quite sure what they do with them all… One might say they’re like bees collecting pollen, really.
Bohemians: The Romantic Reprobates. They deal in nostalgia, art, and scandal. This has brought them surprisingly close to the Bazaar. Why does it tax their Love Stories so heavily?