I think there's a bit of confusion baked into the question, which makes for an interesting discussion. There isn't a single, original 'The Dark Crystal' novel that serves as the primary source material. The universe started with Jim Henson's 1982 film. So, talking about main characters from a 'novel' usually means one of the many expanded universe books.
If we're talking about the core story from the film, which several novelizations cover, the mains are definitely Jen (the last surviving Gelfling, on a quest to heal the Crystal) and Kira (the other Gelfling he finds). The villains are the grotesque, life-draining Skeksis, with the Chamberlain (SkekSil) being a particularly memorable schemer. Then you've got the gentle, mystic Mystics, Aughra the astronomer, and Fizzgig, Kira's furry ball of protective rage.
But if someone's asking about a novel, they might mean J.M. Lee's YA prequel series, which is fantastic. Those books follow entirely different Gelfling like Rian, a Stonewood guard, Naia, a Drenchen, and their friends Gurjin and Tavra. They're the central figures uncovering the Skeksis' conspiracy long before Jen's time. So the answer really hinges on which book they've got in their hands. The film novelizations and the prequel novels have almost entirely separate casts, which is pretty unique for a franchise.