Due to the White Dove cult experimenting on in her childhood to see if they could make her into a weapon, Mary was traumatized by the event, causing her to become withdrawn into herself and stripping her of the ability to show emotions. As a result, Mary has a seemingly quiet and cold personality, rarely showing any feelings or emotions, even when she is speaking. Because of the flat way Mary talks people often think mistakenly think she's being sarcastic resulting in her having to sometimes go out of her own way to explain herself. She is also the only one out of her sisters who gives Leonard the hardest time when they first met, At first, she was suspicious of the latter for not believing his kind nature to be genuine, although she comes to respect him for his never giving up and considers him a hard worker.

Despite seeming cold on the outside, Mary is a very caring and empathetic person. Mary will go from zero to a hundred when people bad-mouth her sisters, The monks or Leonard, This stems from her respecting and being grateful towards Leonard and the monks.

Mary has a deep hatred against the white dove cult, for what they did to her as a child. After Leonard attacks the White Dove cult and rescues the sisters, Mary overcomes her trauma and proclaims herself Leonard's wife and then on she starts acting like a dog following around its master, showing attraction towards Leonard. Her love for Leonard grows so much that, she starts to go into her mating season and tries to bait Leonard into sleeping with her on several occasions.