Lina Mayfleet is the main female protagonist of the Books of Ember series as a whole, particularly the first book, Book 1: The City of Ember. She is 12 years old in the book, and in the film adaptation she is played by actress Saoirse Ronan, who would have been 13 during filming.
Lina is an orphan, raised under the care of her grandmother, known only as "Granny", who is suffering from some form of dementia, likely caused by Alzheimer's disease. Her mother died giving birth to Lina's younger sister Poppy Mayfleet, and her father's cause of death is ambiguous but claimed to be due to a plague that swept through Ember.
In Chapter 10 of Book 1, Granny dies, either due to the dreaded "coughing sickness" or finally succumbing to her mental illness, leaving Lina and Poppy completely alone. They are brought into the care of Evaleen Murdo, a neighbor and friend of the family.
With Doon's help, Lina finds a way to escape Ember, and they find a tunnel that stops directly above the city, where Lina adds to the note Doon had meant to send to Clary Laine, then they stuff it and a rock into Doon's shirt and drop it out of the tunnel, letting Mrs. Murdo find it.