An Extra Stole the Male Leads

Reborn as Selonia Bessin, the heroine of a romance fantasy novel, a girl is thrust into a demon-slaying mission with the novel's three male leads. They successfully defeat the demon lord, but Selonia falls unconscious for three months on the journey back. Upon awakening, she discovers the trio is now enamored with Grace Benet, an unknown extra. Moreover, the supposedly defeated demon lord is back in hot human form! Why has he returned, and who is this mysterious Grace stealing the limelight?