Her parents were spellcasters who were tormented by the demon Despondency, originally the eigth sin, but was now apart from his brethren, the Seven Deadly Sins. It got hold of Akiko, Masumi's older sister, and bound itself to her lifeforce. The demon killed Masumi's father, before her mother could banish the demon from Akiko's body and into her twin swords. With the demon gone, Akiko died.
Masumi inherited the twin swords from her mother. The demon was still bound to it.