[…] A fairy's child (line 14)
This is the first hint that the lady is in some way associated with the supernatural. This line doesn't make it clear whether the lady is a fairy, or whether she's just so beautiful and mysterious that she seems like "a fairy's child."
For sidelong would she bend, and sing A fairy's song (lines 23-24)
Again, it's possible to read this as romantic hyperbole, or overstatement. Is she really singing a magical, "fairy's song," or is her voice so sweet to the enraptured knight that it sounds magical to him?
She found me roots of relish sweet, And honey wild, and manna dew, (lines 25-26)
Giving the knight "manna" makes the lady seem almost divine. "[M]anna" is the heavenly food that the Israelites ate after they escaped from slavery in Egypt, according to the Jewish scriptures. But it's possible that the knight's perception of the honey, roots, and water she gave him is colored by his obsession with her. Maybe it was just ordinary spring water, but he was so in love, that it seemed like heavenly "manna dew" to him.