Gray’s mother.
Ten years ago, when Gray suddenly transformed to have the appearance of that past hero, she immediately fell to her knees in worship. Seeing in her a fervor that far surpassed that of the other villagers, and considering her closeness to Gray in general, the leader they referred to as Grandmother entrusted her with the dagger Erosion.
Gray’s desire to have someone dislike her face was greatly influenced by her mother. To see her once loving mother suddenly seem to forget everything and turn to her as an icon of worship must have been a repulsive sight.
As was described in the story, it was all a facade.
In order to prevent her daughter from being overtaken by that past hero, Magdalena fought against the traditions of the village for half of her lifetime. Even if it meant her daughter would hate her, even if her daughter would be consigned to a life of lonely despair, she was prepared to do anything and everything in her power to stop that from happening.
Furthermore, the one who taught her how to interfere with the village’s ritual was Heartless, who was studying it from the outside. In exchange for information that only an insider could possess, he taught her how to break the ritual.
Later, using the results of that research, he attempted to use Faker as a vessel to reincarnate the Divine Spirit of Iskandar.

Lord El-Melloi II Case Files material: Lord El-Melloi II Case Files Glossary