Just like the title says. To give a bit of background, the group in particular is a group of feral cats with human intelligence. Only a few of the cats ever step onto the surface world (And only do so at night), and the rest live deep in the caves. Their homes aren't total darkness, but most of their time traveling the caves are pitch black. They don't leave the cave for religious and cultural reasons.
They simply don't leave the caves since their ancestor's ghosts are in the caves, and they aren't on the surface. The surface contains humans and predators, and they don't have their ancestors to defend them there, while the caves are safe and familiar, with their ancestors to guide them. They essentially worship their ancestors, and to some extent, the night/darkness.
We're also handwaving any complications that come with living in caves. They are a bit between cats with human intelligence and humans in a cat's body, think the "Warrior Cats" series by Erin Hunter with more uses of body language. They have a few physical differences from cats in our world, but not a massive amount, mostly having slightly opposable thumbs (They can grip objects in a pinch, but don't expect them to hold tools, and they prefer just using their mouth), and sitting on their haunches like squirrels for brief periods of time.
They don't interact with humans, and while some few selected cats leave the caves at night for trade and if the cave cannot sustain the cats fully, that would complicate this question. The condition of the caves greatly varies, due to them being semi-nomadic and moving spots often so they don't collapse the ecosystem of the cave. But, the caves are often windy, and they usually try to set up camp near a water source like an underground stream.
For a bit of a cultural starting point, they are low context in communication (Very straight-forward, as you need to be when you can't usually see the body language and expression of others to infer), communitarism (Individualism isn't prioritized, group responsibility for choices, and emphasism on cooperation, and they primarily make leadership decisions with the help of their ancestors (The leader is actually just a cat trained to decipher signs and omens from their ancestors, as well as communicate with them, their ancestors make the majority of major decisions, and even minor decisions usually have the input of living elders). And, as a note, the ancestors are held on such a pedestal that disrespecting them horribly is enough to get you exiled from the caves. I originally planned on them placing a taboo on eating bats, believing them to be their ancestors and the night's messengers, but another question has brought to light that bats will likely be a heavy food source of theirs.
And, if appearance plays any role, they are small but stocky cats with large ears, spectacular night vision, powerful haunches, larger than average claws, and dark pelts. They also have quite a few cats with the ability to fish, and look vaguely like a Singapura cat, without as small of a size and without all the genetic defects and pelt color.