Cahir, whose full name is far too long to write out, was a Nilgaardian intelligence officer who was introduced in Blood of Elves. There's a lot of mystery surrounding this character. He is loyal to Nilfgaard, but not unreasonably so. He was given a task but ultimately saw it through, not wholly out of loyalty, but because his heart had been caught in his mission.
It's no surprise then that Hissrich describes him as being conflicted and pure of heart, among other things. She goes on to describe him as being a knight in shining armor caught in a harsher time, when the hero doesn't always get the girl. He's a character that must learn what it means to be heroic and brave.
This may come as a shock, maybe even a bit of a disappointment to fans of the video games but Triss isn't actually a major character in the books. She is introduced in Blood of Elves and at one point does prove herself in being a major influence for Ciri. She's a friend of Yennefer and Geralt but it was the video games that expanded on her story and really turned her into a serious love interest for Geralt.
Hissrich describes her as being idealistic, insecure and spunky. There seems to be an attempt to compromise between the character of the video games and the novels here. The games were able to recreate Triss as a major love interest by giving Geralt amnesia, so it'll be interesting to see how the Triss of the Netflix series will tear Geralt away from Yennefer, to whom the witcher is bound, often in more ways than one.
You cannot have a Witcher series without the Continent's most famous troubadour, Jaskir, better known as the slightly eccentric but poetically gifted Dandelion. This womanizing bard travels the world, often alongside Geralt, his closest friend. He may not be as battle-hardened as the monster hunter, but Dandelion does not shy away from adventure and thanks to his wit, he is never completely helpless.
The game depicts Dandelion as being something of a fool and Hissrich's description seems to be leaning in that direction. She describes him as hiding "behind the exterior of an idiot" despite being incredibly intelligent and highly aware of the cold truths of this world. It isn't too far from the Dandelion book fans have come to know.
Ciri's story is a complicated one, far too complicated to explain without some major spoilers. What you need to know about her is that she's the daughter of Emperor Emhyr Var Emreis. While an important part of her story was revealed in The Last Wish, Ciri was properly introduced in Sword of Destiny. The relationship she has with Geralt is a complicated one, but in essence, she is like a daughter to him, which fans of the video game will already know.
The recent tweet described Ciri as brazen, resilient and relentless. She's a growing child with more power than she perhaps realizes. Hissrich describes her story as being one of facing fears and finding a family of her own that will stand beside her through the harsh moments of her journey.