The following character concepts may be of limited use outside of Planescape (or, at least, a campaign in the outer planes), but some could be adapted easily enough to prime material settings. I hope to provide one for each of Sigil's fifteen factions. Part 1 describes four such characters.
The Burned Lady (N/half-elf/ghost) - Believed to have been a priestess of Aoskar who burned to death during the Lady of Pain's destruction of the god of portals, Athar clerks claim to sometimes encounter this apparition deep in the book-filled catacombs of the Shattered Temple. She appears as a translucent figure in a tattered but elegant blue gown, wreathed in a perpetual, incorporeal flame that lights the dark passageways she wanders.
Those who work the deep stacks claim to find tomes (always relating to legends of godslayers and their methods) pulled from the shelves--their text marked with corrections singed into the vellum pages. Factol Terrance believes that witnessing the death of her deity, seeing the suffering of her fellow worshipers, and losing her own life on account of her faith, instilled in her a deep and abiding bitterness toward the lying gods. According to him, she shares her supernatural knowledge in the hope that it will help the enemies of the gods to expose or even destroy them.
Believers of the Source
Padmini Singh (NG/human/bard) - Singh is a philosopher from the prime who stumbled into Sigil quite by accident. She found the city immediately intoxicating for its richness of thought; a place where even the lowest untouchable spent their free time wrestling for purpose and meaning with a sophistication and sharpness and passion that rivaled even the greatest of her colleagues back home. Encountering the various factions, Padmini struggled between the Dustmen and the Godsmen--the former for their ascetic beliefs, and the latter for their emphasis on transcendence through philosophy.
She ultimately found the Dustmen's notion of True Death too distasteful, and joined the Godsmen... albeit with her own ascetic interpretation of their teachings. Unlike most members of her faction, Padmini believes that a spirit's progression through the stages of existence leads not from the lowest grub to the highest of gods, but the other way around. Through divesting one's self of emotion, desire, and worldly concerns, Padmini believes that a being can shed the complexities of mortal life and be reincarnated in a simpler, more pure, more naturally peaceful form. She sees the innocence of so-called "lesser" creatures as a manifestation of calm and wisdom--a release from the rigors of superfluous need and want, and from the constant struggle to *know* one's purpose.
Echelaos (CG/aasimon/fallen proxy) - Once a proxy of Zeus, Echelaos' empathy was a constant whisper of doubt when following the orders of his vengeful and seemingly petty master. The task which troubled him most was being sent to observe the defeated enemies of the Olympians and confirm their continuing punishments were sufficiently cruel.
During one such excursion, Echelaos felt compelled to show some small kindness to a poor soul named Sisyphus, who the gods had condemned to repeatedly push a boulder up a mountain slope only to watch it roll down again, and again, for all eternity. He joined the damned man for a time, putting his own shoulder into the stone to ease the load, and during the climb the two spoke. To his surprise, Echelaos discovered that Sisyphus was not only resigned to his fate, but genuinely content. The former king explained that his entire life had been a struggle--against rivals, plotters, sycophants, and ultimately the gods and death itself. He said that while he first found his fate truly and deeply gruesome, eventually his bitterness began to fade away into mere memories from a part of his existence that was now centuries past. Sisyphus, it seemed to Echelaos, had been liberated from the drive to find purpose. He had realized that all the seemingly important matters that plagued him were ultimately no less futile than his current work. Freedom from hope brought with it an end to all agonizing fixations on what could someday be, leaving him, for once and all, to dwell in *this* moment alone.
Echelaos never returned to Olympus. Instead, he came to Sigil. Now his empathy is invested in caring for the sick and elderly at one of the Cabal's many hostels. He seeks to learn how to embrace the ephemeral relationships that he builds with people who soon will pass away. Most of all, he drinks.
Crumbling Jayk (CE/tiefling/fighter) - The particular manifestations of Jayk's plane-touched ancestry are perhaps poetic for an acolyte of entropy. The tiefling's very touch is corrosive to metal (equivalent to a rust monster). Instead of sweat, Jayk excretes a dry rust that stains his clothing and hair--the whites of his eyes, his teeth, the grime beneath his fingernails... all are tinged with the reddish hue of a cast-off bit of iron left unloved in the rain. He feels a kinship with rust monsters, and keeps two of them (Nail-Biter and Hungry Hacksaw) as beloved pets.