Marie and Ida began following Lief, Barda and Jasmine after they left Dread Mountain and continued to follow the group for days, pretending to be wood mice. They eventually moved ahead of the group and staged a trap, taking the forms of young twin sisters. They leaped into the River Tor, pretending to have fallen from their raft and Ida cried out Marie's name in fear, alerting Lief, Barda and Jasmine.
Lief rescued Ida while Barda dove to rescue Marie, who had fallen in deeper water. Marie pretended to have drowned as Ida wailed, asking to be taken away from the sight. Jasmine began to lead Ida away as Barda attempted to resuscitate Marie. Before the two Ols could fully separate the group, however, Ida was killed by Dain, prompting Marie to throw away its disguise and attack Barda. Lief, with Dain's help, managed to stab Marie in its heart. The Ol died immediately, dissolving into the sand while convulsing into the various forms it had taken throughout its life.
Marie and Ida were the first Ols that Lief, Barda and Jasmine knew that they had encountered, making them more cautious of their surroundings in the future. Dain managed to gain the group's trust because of his help in killing Marie and Ida, furthering one of the Shadow Lord's numerous plans.
Marie's primary form was that of a young girl, seemingly around five or six years of age. "She" sported straight brown hair, a heart-shaped face, long, curling black eyelashes, and golden skin. The Shadow Lord's mark was disguised as a small, brown birthmark on Marie’s left cheekbone. Marie wore a plain, white dress and was an identical twin to Ida.
As an Ol, Marie could change shape into any living creature and it took the form of a small wood mouse for some time. It is implied that the Ol had also shape-shifted into a bird at some point in its life. Marie's true form was that of a tall, white ghoul with the Shadow Lord's mark at its centre. It had a large, peaked head, burning read eyes, and a black, toothless maw.
Marie and Ida were reasonably clever and worked together quite well. Marie did not react when Ida was slain, instead taking the opportunity to try and complete their task. It was a loyal servant of the Shadow Lord and fought ferociously against any who would stand against its master.
Marie was a Grade One Ol, capable of shape-shifting into any living creature, but its abilities are far from perfect. It could not eat or drink and was unable to hold its shape for very long before suffering from the Tremor. It was also unable to hide its unnaturally cold body, though Marie and Ida were clever enough to feign drowning in cold water, giving them an excuse. In its true form, Marie was immensely strong, able to lift and choke Barda. Its cold body was effectively weakened Barda for some time after he was attacked. Marie was a talented enough actor and schemer to fool Lief, Barda and Jasmine. Marie’s weakness was its heart, which was located in the right side of its chest.