Geralt of Rivia
Geralt is the main character of the game and the player has no other choice than to play him. Geralt of Rivia is a witcher, a mutant who survived several "trials" while becoming a witcher, and his white hair is the result of the "Trial of the Grasses" which only a handful of witchers normally survive.
However, Geralt is also known as Gwynnbleid, which means in the elder speech of the Dryads "The White Wolf". When the witchers found him in the surroundings of Kaer Morhen, Geralt couldn't remember anything. It is up to the player to lead Geralt through the game and find out a few things about Geralt's past with the help of some of his friends.
Eskel
Eskel is Geralt's best friend, they grew up together in Kaer Morhen. Eskel is a calm and reasonable witcher and he only appears in the prologue of the game within Kaer Morhen.
However, he is playing a more important role in the official module "Price of Neutrality", where he has problems with Deirdre Ademeyn and fights side by side with Geralt. Here also the question where Eskel got his ugly scar from will be answered.
Vesemir
Vesemir is the oldest and most experienced witcher in Kaer Morhen. He is like a father to Geralt and the other witchers. When Geralt starts the game in Kaer Morhen, it is Vesemir who introduces the necessary skills to him and also some important facts about another witcher called Berengar, who is part of a quest.
Yaevinn
Yaevinn is an elf and the leader of the non-human rebelling group called "Scoia'Tael". He is playing an important role in the game and Geralt can decide to join Yaevinn's "Scoia'Tael", fight against them or stay neutral. As an elf Yaevinn often speaks in verse or metaphors like all of his kind and he treats Geralt with hesitant respect.
Shani
Shani is an old friend of Geralt and she is playing a halfway important role in the game. She is a healer and will always heal Geralt when he's in her reach. Geralt needs Shani to perform some special quest parts.
While Geralt can't remember her until a special point in the game, Shani is still loving him. When it comes to the party in Shani's house, Geralt will hear about some details of his past. Shani is also one of the two women who will watch over the child Alvin later, depending on Geralt's decision to whom he will bring this child.
Triss Merigold
Triss Merigold is a sorceress and plays the same important role as Shani, she is the second choice for Geralt to find a place for the child Alvin. She is wounded in the prologue and Geralt has to learn about potions by healing her with one. Triss also reveals some things from Geralt's past, but not very much. She is member of King Foltest's Council and mainly interested in politics.
Dandelion
Dandelion is a bard and one of Geralt's closest friends. There is not one situation Dandelion has no silly verse for, however, his ballads are well known throughout the land. He is also known for having fast feet when running away from a women's husband...
At some parts of the game the player has the chance to hear such a ballad, Dandelion doesn't hide his skills when writing a new ballad about one of Geralt's adventures. He also knows many things about Geralt's past, even if he's telling that parts in his own way.
Zoltan Chivay
Zoltan Chivay is also a close friend of Geralt, even if Geralt can't remember him. He is a dwarf and his first appearance in the game is when Geralt approaches a fighting scene in the Outskirts of Vizima where racist thugs are attacking Zoltan. Zoltan is very pleased to see Geralt, because he saw Geralt dying with his own eyes five years before.
Geralt learns about Dice Poker from Zoltan after this encounter and Zoltan also tells Geralt about the racists and the "Scoia'Tael". He can be chosen as a companion for Shani's party later.
Siegfried of Denesle
Siegfried is member of the Order of the Flaming Rose and Geralt first meets him in the sewers under Vizima. Geralt can decide to fight with him against a cockatrice there or fight alone. Later in the game Geralt has to decide to fight against the Order, with them against the "Scoia'Tael" or to stay neutral instead.