Mercedes's biological father was the head of House Martritz of the Adrestian Empire, but he died when she was very young, leading to the house's dissolution. Her mother then remarried Baron Bartels, to whom Mercedes' brother Emile was born. Mercedes and her mother both found themselves neglected in favour of Emile, who was a crest-bearing heir, so they eventually fled to a church in Faerghus.
She would later attend the school of magic in Fhirdiad, where she became close friends with Annette and also met Lorenz of the Alliance's House Gloucester. Entering the Officer's Academy at the age of 22, she is older than most of the other students, appearing mild-mannered and pious. While this is true, Mercedes is strong-willed and unafraid to speak her mind, especially to men who would underestimate her.