Lissandra the Gate-Seeker is invariably one of the first NPCs that PCs will run to if the GM has decided to move a campaign from the Prime to the Planes. Which probably means the most common words she’s ever spoken in just about every campaign like that is “[Insert name of campaign world here]? Sorry cutters, never heard of it.”
One of the top movers and shakers in Sigil, Estavan the Merchant Lord is the head of the Sigil office of the Planar Trade Consortium.
Long ago, there was a deity known as Aoskar who was the god of doors and portals. This made him popular amongst planewalkers, whose lives and livelihood depends on the passages between the planes. He eventually become so popular (and hence so powerful) that he began to threaten The Lady’s rule over Sigil. While her control over the City of Doors remained absolute, planewalkers would whisper prayers to Aoskar as they passed through portals in and out of Sigil – portals that were created and destroyed at the whim of The Lady of Pain. But she made no move against Aoskar, until one day one of her own servants left her to join Aoskar. When one of her dabus took the robes of Aoskar’s clergy, it was the final straw. The Lady’s retribution was swift, terrible, and complete. Aoskar’s temple was razed – it is now known as the Shattered Temple, headquarters of the Athar. All of his clergy in Sigil were flayed alive. She once again asserted her complete dominance of Sigil by killing Aoskar before casting his dead body out of Sigil to the Astral Plane and instituting her command that no deity ever be allowed within the confines of the city again. As for the traitorous servant – she had a special punishment in mind.
Wooly Cupgrass, the loopy bariaur Sensate with a taste for potions and poisons, can make a great resource for PCs in Planescape. He can identify, buy, and sell potions, elixirs, and poisons, and he’s also a great source of comedy for the GM.
Iarmid, owner and proprietor of The Other Place, is a fantastic NPC for introducing new players to the Planescape setting. Throw them into Baator in the To Baator and Back adventure from Well of Worlds, then have Kylie or another tout bring them to The Other Place when they stumble into the Great Bazaar covered head-to-toe in dust and dried blood. From there, Iarmid and his assistants can answer all their questions about the planes they stumbled into and this strange city they find themselves stranded in.
Kylie the Tout is featured in Uncaged: Faces of Sigil, as well as Doors to the Unknown and some other Planescape adventures. I’ve used her extensively in my own games as an easy way to indicate to players when a message/invitation/etc. is really important: if the message is delivered by some no-name tout or factioneer, sure they’ll read it but they may save it for later. If whomever sent the message went to the trouble and expense of having Kylie deliver it, they’d better believe it’s important!
In the Abyss was the second full adventure published for Planescape, and firmly established the Doomguard faction as one of the bad guys. While the Sinkers were a part of The Eternal Boundary, they were as much a victim of circumstances as all of the other factions involved. In the Abyss put the Doomguard’s alliance with the tanar’ri front-and-centre in the form of the Ship of Chaos. It also for the first time put the player characters square in the middle of the eternal conflict between the tanar’ri and the baatezu, and forced them to make a major choice: honour their contract with the baatezu to turn the Ship of Chaos over to them, or disable the ship to keep it out of all fiendish hands. Of course, if they’re not quick enough, they might end up lucky to escape with their lives!
Updated 31 May 2015: Added The Fourth Door: A Reality Beyond
Yet another adventure anthology, Doors to the Unknown involves the players in the politics of Sigil more than most other adventures. There’s a nice strong link between each chapter as well as a rough timeline that allows the GM to put other adventures in between each chapter. There are some weaknesses in the adventure (for example, there’s no indication that the players need to take the Rod of Mercury from the gearwork golem in the Thuldanin junkyard) but they’re nothing a good GM can’t work around.
A Pathfinder Planescape update of Saure from Uncaged: Faces of Sigil.
Omott, the naïve linqua from Uncaged: Faces of Sigil, updated for Pathfinder Planescape.