Maria appears as a tall masked woman wearing a long fancy black dress. The mask is ornate with what looks like a butterfly or four flower petals and two long strings. The dress is decorated with roses on the top. She appears to have a corset and the dress reveals part of her chest. She wears lipstick.
When deceased, her skin-tone is greyish and cadaveric.
At some point in the past, Maria became an exorcist and used her innocence to fight akumas. She died and Cross Marian somehow ended up in possession of the corpse which he uses magic to take control of.
Noah's Ark Arc
Maria's body is seen for the first time summoned by Cross in the battle against Tyki Mikk. He first orders her to use her song to conceal the young exorcists and then to hinder Tyki's perception.
Invasion of the Black Order Arc
Powers and Abilities:
Singing: As her innocence requires it, Maria is an excellent singer.
Maria possesses an unnamed parasitic-type which takes the form of her vocal cords. She uses the melody produced with those cords that affects the discernment and freedom of her victims 
- Magdala Curtain (聖母ノ加護 (マグダラ・カーテン), Magudara kāten), Maria uses her song to create illusions that can conceal the presence of others.
- Carte Garde (脳傀儡 (カルテ・ ガルテ), Karute garute) which allows her (or Cross) to control the person's movements by affecting the brains of both humans and Akuma.