In the Elizabethan play The Birth of Merlin, the daughter of Lord Donobert and a maiden at Aurelius Ambrosius’ court.
She was betrothed to Cador of Cornwall until her sister, Modestia, convinced Constantia to join her in a nunnery.
Source
The Birth of Merlin, or the Childe Hath Found His Father | Attributed to William Shakespeare and William Rowley, 1662