Thywinn is a Pirate Witcher.

He's a Bastard Prince. He was taken as a child in a failed attempt at a hostile takeover of his kingdom and the rebel kidnapped him instead sense his father would rather keep the kindgom under his own rule than let the rebel take over the kingdom. So the rebel sold off Thywinn to the School of Cat (A Witcher School) and grew up as a Assassin/Spy/Merc

He relies on his cunning tactics, swift feet and charisma but usually prefers stealth over confrontation

One day, his base of opperations was attacked with all his allies by a invading force of royal armies due to the fear of this specific school of Witchers would kill important members of their royal family. (School of Cat decided to use their Witchers for assassinations and spy tactics for any kingdom willing to pay them.) So in the defense to have a last stand against the royal armies, Thywinn realized they were all going to die. So instead of saving as many allies as he could and helping them until the end, he bailed last minute leaving his allies to die

And when he decided to go back to his royal origins in the hopes that his family would take him back and accept him, his brother (Both Thywinn and his brother loved eachother whereas Thywinns parents are cold to both of them) stopped him in his tracks. But Thywinn convinces his brother to let him see his parents against his better judgement. Thywinn's parents ended up hating that he was a Witcher and bastardized him. But they also ordered him to be killed on the spot by the guards as to avoid political turmoil and a possible tainting of the royal bloodline getting out there. So Thywinn ended up killing both his parents and leaving his brother alive. He then ran away yet again and has to live with his choices for the rest of his life.

So his younger brother (The prince who Thywinn spared) takes the crown. And in a lust for revenge and the misguided idea of making their dead father proud, Thywinn's brother hunts him down with his royal armies and bounty hunters. His biggest reason for hunting Thywinn down is for betraying the family, but for political reasons it's because he is a King Slayer now. Thywinn now must survive the onslaught of his brother's armies and power while also learning the true meaning of family with his rag-tag team of party members he will gain along the way. He also wants to do everything in his power to avoid killing his brother and making him forgive Thywinn so he can live a peaceful life despite the constant effort his brother will put in killing Thywinn.