Eilonwy is a member of the Royal House of Llyr, and the women in her line are formidable enchantresses, including her mother, Angharad, and grandmother Regat.

As a member of the Royal House of Llyr, she wears a pendant depicting a silver crescent moon, the family emblem.

Eilonwy commonly uses unusual similes and metaphors, such as "If you don't listen to what somebody tells you, it's like putting your fingers in your ears and jumping down a well."

Princess Diahan from Lloyd's earlier novel Time Cat: The Remarkable Journeys of Jason and Gareth, who served as a prototype for Eilonwy, was likewise described as wearing a white robe and sandals.

Eilonwy is voiced by Susan Sheridan in the 1985 Disney animated film The Black Cauldron, which is loosely based on The Chronicles of Prydain.

When they are captured by the Fair Folk, she convinces the king to give them Hen Wen, provisions, and a guide to Caer Dathyl.

We later discover that she is the last living descendant of Llyr Half-Speech, the Sea King, which is why Achren abducted the girl as a small child.

The Castle of Llyr, the third novel in the series, reveals that this is really the Golden Pelydryn, a magical artifact passed through Eilonwy's family from mother to daughter.

The events of that book bring Eilonwy back to her ancestral home, Caer Colur, and give her a greater sense of her ancestry.

The Disney film depicts the bauble as a semi-sentient object that floats through the air under its own power, but in the books, it is described more like an orb of gold that must be carried.

The fourth book in the series, Taran Wanderer, sheds light on the ultimate fate of Eilonwy's mother, Princess Angharad.

Taran makes this discovery while Eilonwy is being fostered at the foreign court of King Rhuddlum and Queen Teleria, learning to be "a proper princess".

Among the stories included therein, readers learn about the circumstances that resulted in Princess Angharad running away from home to marry the commoner Geraint.