Who is Elvira de la Cruz from Purity of Blood and what is their importance?

Elvira de la Cruz is a captive in the convent of Adoratrices Benitas, where she is being held by Fray Coroado, Elvira is later delivered to the Inquisition, where she is tortured, and as a result, implicates Inigo Balboa in heretical conspiracy and recants. She is then burned at the stake.