Until the first half of the story, we don't know if Veryan the Fairy really exists. Many people imagine that she is just a vision of Syndag's mind, because she can be seen only by the boy. However, in the caves of Gronaem, she shows her powers to protect the fellowship from the evil spells covering the Sword. From that moment on, Hasius and Midar know for sure that Veryan really exists.
She also knows that Syndag has demon blood in his veins, but she allows the boy to discover this by himself.