She seems to have been Roestoc’s ruler. She fell in love with Gawaine and gave him a girdle and locket when he defeated Sir Segurades for her. Gawaine subsequently gave the girdle and locket to the sister of Helain de Taningues.
Vulgate VII names a ‘Helyes li chatelains de Roestoc’. The “li” is masculine, and Helyes appears to be one of the rebel kings, possibly killed in battle, which makes the Lady of Roestoc either his widow or his orphaned daughter.