Marit was a member of the Volturi guard, and one of few members without possession of supernatural capabilities. Her useful skills come from her superior physical strength and battle techniques. Aro finds her very useful like Ettore and Felix, who are also not gifted.
Little is known about Marit's history other than that she was created by Aro and had been within his guard for centuries. She was kept within the guard due to her battle techniques like Felix and Santiago. She was sometimes jealous since Felix was proven the strongest vampire the Volturi have encountered, but she always remained calm and maintained a permanent position in the guard.
She was briefly seen in New Moon when she saw Felix and Demetri escorting Edward, Alice, and Bella. She stood still like a statue and watched Edward pick a fight with Felix to prevent the guard from killing her due to her knowledge.
In Breaking Dawn, Marit was present in the throne room with Corin, Demetri, and Felix as Irina came to report a "crime" about the Cullens for creating an immortal child, which later turns out to be a great misunderstanding.