He was not sure how he had ended up in this land. Years ago, after suffering grievous wounds as a child, he had fallen through a portal while stumbling in the snow. It sent him to the continent, where he had the misfortune of being found and captured by the mad mage Vilgefortz.
Vilgefortz, after healing him, sensed something odd about him and decided to use him as a test subject. From the ramblings he could remember, Vilgefortz thought he possessed odd magic—magic that could not be used normally, like a regular mage. He screamed endlessly of ice and fire.
So Vilgefortz decided he would draw the magic out in a way that all who were not natural sources could use magic: The Witcher mutations. But Vilgefortz was quite egotistical, remembered the Witcher; he could not just use the regular Witcher mutations. No, he wanted the perfect Witcher for his experiment. So, over the next several months, he hunted down all the mutations he could find. The Bear, the Griffin, the Cat, and the Manticore were all he managed to find. The Wolves kept theirs well hidden, and the Viper... "Even I don't know much about them," thought the Witcher.
Then came the pain, the Witcher thought as he went through the several mutations. Vilgefortz may have been mad, but he truly did perfect the mutations: the strength and size of the Bear, but perfectly balanced by the speed of the Cat; the toxicity resistance of the Manticore; finally, the magic sign power of the Griffin. White bled into his hair, much like his old mentor, but his eyes were oddly purple, unlike any other Witcher.
Of course, it still did not have the effect Vilgefortz wanted, so the Witcher stayed locked up in the dungeons of Stygga Castle. Eventually, he was rescued when Geralt made his desperate attempt to save Ciri. He was found while they were there and luckily was able to make it out before the castle collapsed. He was then trained by the Wolf Witchers for years until he was let out on the Path.
He stayed on the Path until recently, in an old Elven ruin where he was hunting a very old Katakan. The fight went on for ages until the Katakan somehow managed to activate a portal, taking them back to this forsaken land.
"Now I know why Geralt hates portals so much," thought the Witcher.
For the last few months, he had been hunting bandits. "It's not like I know anything else. Kaer Morhen didn't exactly teach me how to cook a gourmet meal," chuckled the Witcher.
Now here he was at the Red Keep to meet the King. "Best try to get the gold and get out of here," thought the Witcher.
However, all thoughts of gold went out the window when he stepped through the door and saw her.