The bastard and the princess

It is a well known fact that the king of the south have many mistresses.
But very few know that he have a bastard son.
Hidden from the public, Tristan is trained as a spy, mastering several languages, material arts, and a lot of things you have yet to discover.
But what happened when he is sent to the Kingdom of the North desqused as a merchant?
And why dose the kings youngest daughter Isobel, look at him like she can see right through him?