Arianrhod, the heroine of the story is a Celt. The origin of the name "Arianrhod" is from the Celtic myths and means "silver wheel" or in other words, the moon, so Arianrhod is the name of the moon goddess. It is said in the story that Arianrhod's people are descendants of the tribe of Danu, the mother goddess in Celtic tradition.
Those who are familiar with Celtic myths would also expect the characters to follow the "Rule of Names", that is, everyone has a true name and the name one goes by. Never reveal the true name to anyone. Those who know the true names of others have power over them.
No one knows Arianrhod's true name, not even her closest friend Enlutha. Her true name was given to her by Legion, a warrior in shining silver chain and mail armour, who came from the Crystal Palace.
No one has power over Arianrhod unless they know her true name.