Mary is a young girl with short hair. As a slave, her hair is unkempt and her face is scruffy. She wears a simple frock which reaches upto her knees. A buttoned sweater with a soft collar is worn over it. The sweater has a pocket with a flap on the bottom right of the front.
Although she was sold as a slave at an early age, Mary has an extremely optimistic attitude. Despite frequent scoldings and physical abuse, the young girl manages to keep her positive outlook on life. She has great faith in her elder sister and believes that everything is fine as long as they are together. Her wish is to become strong like Gray and to always stay by her sister's side. Inspite of her rough life, she takes great pleasure in novel experiences such as the wind blowing across her face while leaning over the side of a ship.
To pay off their parents' debt, Mary and her elder sister were sold off to Cappuccino, who forced the child to work as a manual labourer and refused to educate her.
Working as a slave in Cappuccino's Bar, Mary was reprimanded by one of the waiters when she failed to deliver some empty bottles to the bar. Acknowleding his rebukes with resignation, she began to murmur something under her breath and prepared to complete her task, when she was surprised by a young Gray Fullbuster, who had witnessed the entire scene and offered to help her. Carrying one box of bottles while Gray carried all the rest, the two headed towards the bar. She asked him whether he was fine with carrying most of the load, but he just brushed off her concern.
Finally reaching their destination, Mary thanked Gray for his help and the two exchanged introductions. Gray questioned her about the strange words she was chanting earlier and was told that it was a good luck charm which her elder sister had taught her, proceeding to say that it brought Gray to her in her time of need. Flustered, Gray immediately changed the topic and asked her why she worked in a place which was clearly not meant for children. Mary explained that she and her elder sister were bought to work there by the owner. Gray was astonished at the frivolous manner in which she made such a depressing statement, but the girl retorted that everything would be fine as long as she was with her sister.
Suddenly, the waiter who had abused Mary earlier came flying out of one of the bar's windows, severely beaten up. Peeking through the damaged structure, the two children were astounded to see Gildarts Clive standing in the middle of the room holding a young woman captive. The Mage jumped out of the room, threw the two children and the woman, revealed to be Mary's elder sister Amelie, over his shoulder and began to run towards the docks. Amelie explained that she had requested Gildarts to free her sister from Cappuccino, but he seemed to have decided to flee the town with both sisters.
Mary was reassured by her sister and told to trust their two saviors, when they noticed Cappuccino riding Milk Boy catching up to them. Gray easily foiled the pursuit by using his Magic to make them run into a wall of ice, a feat which Mary marvelled at.
At the Mone docks, the group went aboard the ship meant to help Gildarts find and exterminate Unicol as per the request he had accepted. Leaning over the side of the ship, Mary enjoyed the wind whipping across her face and called her sister to join her. But this blissful moment was short-lived as the ship was suddenly attacked by Unicol, causing Mary to huddle up to her sister in terror. As the ship began to sink, Gildarts picked up everybody on board and jumped onto a platfor created by Gray.
Things were further complicated when Cappuccino and Milk Boy showed up in their own boat to reclaim Mary and Amelie. When Gray proclaimed that he would take down Unicol and leave Milk Boy to Gildarts, Mary was astounded to see his determination to protect them. As the showdown proceeded, Amelie was knocked off the platform by one of the monster's attacks, only to be caught by Mary, who stated that she wanted to be strong like Gray. But she began to slip off too, when Gildarts arrived to rescue them, commending Mary's bravery. The young girl tearfully requested her sister to never leave her as she would not be a burden, something which she was lovingly admonished for and then pulled into an embrace by her sister.
The battle finally ended in victory for Gray and Gildarts. Mary found Gray floating some distance away, too exhausted to move, and pulled him onto the platform, sobbing while praising him. The group then took Cappuccino and Milk Boy captive and continued their voyage in the Mone Lord's boat, Mary sleeping throughout.
They soon reached the western bank, where the Army arrested Cappuccino and Gildarts left the party. Gray travelled with the sisters until they reached the point where their paths diverged. Just when the question of money to fund Gray's journey to Magnolia was raised, the group happened on a field of Gold Cotton. Mary and Amelie harvested it and gave it to Gray. Mary began to sulk when the time to say their goodbyes arrived, but cheered up when Gray promised that they would meet again. The two then took their leave.