The Tarterian Depths are a soulless place, where Evil is the main driving force. Few (bar The Revolutionary League) would ever choose to live on Carceri, but some have no choice - whether they were accidentally Plane Shifted, or fell through the wrong colour pool at the wrong time. Some even came in large enough numbers to have children, even attempt to settle down - though they soon learned to regret their mistake.
Physically, many appear slight and even weak by average standards - but they have an avaricious cunning that more than makes up for it. Most often, they have a cruel caste to their features - a permanent "smirk" like they're going to fuck someone over, but they just haven't decided who yet. Thin mouths filled with teeth that are far too sharp , eyes which seem to glow with the tiniest hint of inner red light, grey-ish or chalk-like skin and minor disfigurements (such as claw-like hands or nails, forked tongues, vestigial tails, heterochromia, warped bodily proportions) are also all features commonly found on Carceri-Touched.
Carceri-Touched are driven by their greed and selfishness. While many people believe they are the centre of the multiverse, your average Carceri-Touched takes this to heart - believing themselves to be the most important as well (after all, how could the Multiverse exist without him?). They don't see other people as "real" - just further aspects of the rest of the universe as it revolves around them. As such, they have no qualms about killing or stealing from people - but believe heartily in revenge upon those who would dare think do the same to them. They are, to their core, Neutral Evil - the exemplars of greed. Many are also somewhat convinced that their existence is defined by suffering - whether feeling it, or sharing it with others.
In Sigil, many join The Fated, abusing the philosophies of their brothers to no end, while some become Doomguard or join the Bleak Cabal. Where they can, they will try and get themselves into positions of power, so that they might gain as much for themselves as they can, and to better cause suffering for it. Many become adventurers out of sheer avarice. or some cruel desire to spread the suffering of their existence out to others.
In Sigil, there is one Carceri-Touched of some importance - Talath Blacktooth, a Carceri-Touched Halfling who has (apparently) just made himself the first Cleric of... himself. He teaches a form of Enlightened Self Interest to those he finds interesting (or wants to rip off later), and encourages the local Barmies to worship him where he can.