Nimue verch Wledyr ap Gwyn, or the "Lady of the Lake" was actually a sorceress from the future who helped Ciri find the way to Stygga castle. She is a graduate of one of the magical academies - most likely Aretuza. As a girl, she took special interest in the history Ciri and Geralt, and in particular their ultimate fate. Over time she began to lose interest in Ciri's story, but an unexpected encounter with a mysterious man re-ignited her interest. In her private tower, she scoured images depicting scenes from the sagas, as well as all the books, fables, legends; every nook and cranny trying to find more information about Ciri's history. She even recruited an assistant, Condwiramurs Tilly, an oneiromancer, who helped with her knowledge of ballads and singing.
Nimue has an elder sister Orla. They both appear as children in a minor scene from the future in the novel Baptism of Fire. In that vignette, it was revealed that Nimue was known as "Squirt" as a child, due to her diminutive size.
Nimue also appeared in scenes from the future in Season of Storms, where she traveled from her home village Wyrwa (can be translated as "Gap" or "Crater") to Aretuza, and "met" Geralt on the way.
Nimue is a name given to many female characters in the Matter of Britain literature. Nimue was the name of the "Lady of the Lake", who gave Excalibùr to King Arthur; the name of the woman who brought the dying king in Avalon; the name of the woman who took care of young Lancelot; the name of the sorceress who seduced and imprisoned Merlin. In all these cases, Nimue is doubtly a human but a mythological creature like a nymph or a nereid.