Mariette was raised in Shadowgate, the northern most town in Deltora, with her older sister, Kirsten. One day, a travelling performing troupe called the Masked Ones arrived in Shadowgate, and both Mariette and her sister became infatuated with Bede, the group's musician. Bede originally fell for Kirsten, but he left her in the end, and she fled into the mountains out of shame and grief.
The following year, the Masked Ones returned to Shadowgate, and Bede fell in love with Mariette. The two met in secret several times before Bede left the Masked Ones to be with her. The two fled into the mountains like Kirsten before them and were thought dead by the Masked Ones and the town.
However, Bede and an exhausted Mariette were guided to a titanic castle near Shadowgate by Kirsten, who had been given magical powers by the Shadow Lord. Kirsten offered Bede the chance to abandon Mariette for her, but Bede remained loyal, and Kirsten imprisoned both of them. Mariette was trapped inside her own locket and used as a hostage to keep Bede obedient.
While traveling with the Masked Ones, Lief was told the story of Mariette and Bede by Bess, the troup leader and Bede's mother. Lief thought of her and her sister again as he, Barda, and Jasmine passed the town of Shadowgate, and pitied her parents.
The companions eventually reached Kirsten's castle and were greeted by her, causing Lief to mistake her for Mariette before she introduced herself. Kirsten, masquerading as Bede's servant, told the group that Mariette was being held captive, all the while touching the locket around her neck to keep Bede in line. However, Bede was able to warn Lief about Kirsten through his music, and on a hunch, Lief stole the locket and gave it to Filli for safe keeping.
Later, when cornered in a snake pit that held the Sister of the North, Lief distracted Kirsten by reminding her that she lost Bede to Mariette. Kirsten boasted that she was better than her sister in every way, which allowed the snakes in the pit to latch onto her hair and slowly kill her. Upon her death and Honora's destruction of the Sister of the North, Mariette was freed from the locket.
Little can be said of Mariette's personality, as she was barely seen in the series and never spoke. She was clearly deeply devoted to Bede, given that she risked death by going into the mountains with him, a love that he reciprocated. She did not seem to have a good relationship with her older sister; Kirsten held Mariette in contempt, and Mariette seemingly had no issue with loving Bede, despite his role in Kirsten's disappearance.