Guinevere is a mythological character in the series mentioned only in legends and stories.
The following information in this article contains spoilers!
She was the only person in her village to trust the dying Gawain, and despite being ridiculed, pledged her undying love for the dragon. Following information from Gwilanna, Guinevere caught Gawain's fire tear, turning the dragon into a boulder. Heartbroken, she placed the tear into a hollow bone vessel and went to Gwilanna. She refused to give the sibyl the tear. Enraged, Gwilanna caused many natural disasters such as earthquakes and tsunamis. Gwilanna followed the two, and created a huge wave that crashed into Guinevere and Thoran. They disapeared, and Gwilanna believed that she drowned them, but the tear somehow survived.
Guinevere and Gwendolen escaped on the back of Thoran, who had befriended Guinevere before. When they finally came to rest at the centre of the northern stars, Gaia told Guinevere to release the tear. The girl spilled Gawain's Fire Tear into the ocean, freezing it and turning the ocean into the present-day Artic ice. Somehow unknown, Gwilanna obtained Gwendolen, which suggests that she did not escape with Guinevere. Guinevere might have died with the first white bear, but this is not certain and research is needed.