Arianllyn was betrothed to Adaon Son of Taliesin. Very little is known of her; Adaon told Taran that she had given him his brooch. Arianllyn's identity, heritage and place of residence are all unknown. We might surmise that she was a bard, for she possessed an enchanted item of bardic power. Perhaps she was a descendant of Menwy the Bard, for it was he who had fashioned the brooch.
We may further guess that she was beautiful, wise and intelligent, for what other sort of woman would Adaon have chosen as his bride? Given the derivation of her name (arian is Welsh for "silver") it's probable she was either silver-haired or gray-eyed, or both.
Otherwise Arianllyn's description and persona remain a mystery.