Maiden daughter of King Assentin.
In a series of interlocking quests, Gawain needed to bring her to the court of King Amoraen so that Amoraen would let him keep the magic Sword with Two Rings, which he needed to trade with King Wonder for a magic chessboard.
Arriving at Assentin’s court, Assentin imprisoned him but gave him to Ysabele for care. Gawain exerted his traditional charm, and Assentin eventually caught them in flagrante and threw them in prison. They were freed by the spirit of a dead knight who Gawain had previously given a burial.
On the journey to King Wonder’s court, Ysabele was abducted by Sir Estor (Hector), but Gawain saved her. Gawain may have later married her.
Source
Roman van Walewein | Penninc and Pieter Vostaert, mid to late 13th century