The ghost of Guinevere’s (Guenevere) mother, in The Awntyrs off Arthure at the Terne Wathelyne, compares herself to an unidentified woman named Berell, claiming to have greater beauty.
Berell does not figure into Arthurian legends anywhere else, though Ralph Hanna suggested that the line originally named Brisen, Elaine’s servant.
Source
The Awntyrs off Arthure at the Terne Wathelyn | Late 14th century