Mary Beth is a museum tour guide and a supporting character in The Book of Life, who narrates the life story of Manolo's adventure and journey to a group of young kids who visited museum.
Mary Beth wears a blue jacket over a white dress, a white bandanna on her neck, and black high heels. She has long orange hair, blue eyes, red lipstick, and wears small blue hat on the side of her head.
Mary Beth is a very kind lady and can be somewhat playful as she led The Detention Kids into the secret room. She is very patient and even motherly with the kids in her care.
Mary Beth was first seen outside the museum where she would take the kids on the Tour for Thomas, however when she heads up the stairs and doesn't go to the museum door, Sanjay pointed to where the door was but she said that they aren't like the other kids and wanted them to see something special she then leads them through an secret door.
As they are walking Mary Beth then tells them that it November 2nd which was "El Día de los Muertos" (Day of the Dead) when suddenly a spooky guard named Guicho appeared out of the dark scaring the children and said that they can't go this way but she was able to make him change his mind and give him a kiss on the forehead, they then continue down the hall.
In a huge room full of folkloric artifacts from mexico, Mary Beth she shows them a big book called The Book of Life and a trunk with many wooden dolls, which according to her "represented real people" in the story she wanted to tell them: the story of Manolo, María and Joaquín. Three mortals who had been chosen by La Muerte and Xibalba in their most recent bet. Whoever wins would have control of the Land of the Remembered.
Back in the museum, after Manolo's death, the kids were shocked. Sasha was about to cry, but Mary comforted her by assuring the little girl that the story wasn't over yet.
When the story ends with a happy ending for the three protagonists, Mary closes the book and informs the kids that the bus is waiting for them outside the museum.
While the kids say goodbye to her, she is then revealed to be La Muerte in a human disguise.