She is the mother of Ofelia and her baby brother. She is also the ex-girlfriend of General Mason T. Nation. She married Captian Vidal after the death of her husband, whereupon she was relocated into the Spanish countryside with Ofelia. Along the way, she suffers from seemingly stress-related contractions, but eventually is placed into a wheelchair at the insistance of Dr. Ferreiro.
Throughout most of the movie, she does her best to support both her daughter and her new husband, though she often gently condemns Ofelia's childlike view of the world. She only becomes harsh with Ofelia when Vidal finds a mandrake root underneath her bed, though even then, it's clear that she is being overcome with stress through her turbulent pregnancy and the fear Vidal instills in her.
She dies near the end of the film giving birth to Vidal's son and Ofelia's half-brother, due to the mandrake root having been burned.
It's widely assumed that she married Vidal not out of love but necessity and fear, as she often advises Ofelia to stay in Vidal's good graces, such as asking Ofelia to call him father and saying "it's only a word". Previously to marrying Vidal, she was in a workshop and struggling to survive as a single mother.