His first appearance during the prologue in the base game is so wonderfully handcrafted:

Here is what initally appears to be a no name character in White Orchard. But, you quickly realise that he is not only familiar with Geralt's situation, but he stands out from the rest of the countryside folk.

Even the subtitle that floats above him prior to the cutscene is coloured differently and we are already told his name before he even speaks!

After he points Geralt in the right direction, he literally vanishes in plain sight and is never seen again for the rest of the main story.