Guinevere was first introduced when Gandalf, Aragorn, Legolas, and Gimli came to Rohan to free Theoden of Sauron's grip. Gifted with both foresight(visions) and eternal youth, she was one of the eldest and most loyal of Theoden's subjects. Previously Theoden's advisor she was removed from the position by Grima "Wormtongue" but remained a loyal subject, even as darkness enveloped Theoden. In addition to losing her position as Theoden's advisor, she became Eowyn's dresser and the private chef for Eowyn, Eomer and Theodred. Eomer was the only one whom knew her true age alongside Theoden, which was estimated to be over a thousand years as she was a guide for the first king of Rohan also. A great warrior, Guinevere was female, so she was unable to go out on the battlefield unless disguised, and used a disguise known as the "White Lady of Rohan". Over the time Aragorn, Legolas, Gimli, and Gandalf spent in Rohan, she developed a close relationship with all of them(apart from Gandalf whom she already knew). Gimli, as she stated, was very brave as he asked her(shamefully) her true age to which she responded "You are brave, dwarf, as to ask me such a question. I am thousands of years old, my young friend." Exactly how old she was remains unknown.
Her parents are said to be the Queen of the Gods, Hera and the King of the Gods, Zeus though it was never confirmed. Evidence has been provided that she has connections to the Gods, as she has been said to have received gifts from the Gods at birth, such as beauty from Aphrodite, prophecy from Apollo, wild animal taming and battle prowess from Artemis, wisdom from Athena, harvest prediction from Demeter, inner harmony from Harmonia and lastly, eternal youth from Hebe. However, such as never been proved.
Guinevere disappeared after the triumph of The Fellowship and was said to have died but it is highly doubted because of several appearances of the "White Lady of Rohan" throughout Middle Earth after the fall of Sauron. Some have said that she joined her supposed parents(Hera & Zeus), others have said that she still exists today.