Angelique: Maren no Rokukishi

The protagonist is an orphan who has no memories of her parents. She is raised by a family acquaintance and acts as the elder sister for their son, Renaud. One day, Renaud is missing and the villagers fear the ensuing wars too much to look for him. When the protagonist searches for him, she learns he has been recruited by Leviath and is one of his knights. The protagonist fears for his safety and decides to stay with the knights. As she does, she learns that she harnesses a mysterious power which helps Leviath's cause and learns about her lost past.