The daughter of a Thracian king who fell in love with Theseus' son Demophon. The young man had ended up there while sailing home from Troy, and the king gave him part of his kingdom and married him to his daughter.
After some time Demophon missed Athens so much that he asked to go home for a while. Phyllis agreed after he had promised he would be back soon, and off he sailed.
When Demophon did not return on the agreed date Phyllis hanged herself in a tree at the spot they would have met. The place was called Ennea Odoi, Nine Roads, since Phyllis had returned there nine times that day to see if Demophon had arrived.
When Demophon did return, he had only been delayed, he clinched to the tree in desperation, only to see that the tree suddenly was covered in leaves and blossom.