A Peacekeeper loyal to Apollyon and the Blackstone Legion, Mercy was often ordered to operate independently of the army. She would be sent on scouting and infiltration missions. While preparing to assault Svengård, Apollyon ordered Mercy to go on ahead to eliminate sentries and sabotage the outpost.
Later on in the story, due to actions that Apollyon committed against the Vikings that forced The Warden to leave the Blackstone Legion, Mercy, along with Holden Cross and Stone, would leave alongside them, forming a new Iron Legion in the homeland of the Knights. They would later take part in the battle against Apollyon by assaulting her fortress alongside the Samurai.
A rather soft-spoken individual, Mercy says very little except what needs to be said. This is because of the isolated nature of her missions. She is very perceptive and analytical, taking very detailed notes about her environment, her enemies, and the people she meets. There's also a very strong chance that Mercy is a sociopath. Given her self-expressed lack of any real investment in the lives of her fellow soldiers, as well as her subsequently expressed desire to actually feel what her colleagues display when the corpses of Blackstone Legion soldiers are discovered by their friends. She also seems to be partially fanatical since she repeated certain overzealous mottoes in an obsessive tone during Sabotage, namely "Doubt is death, death is doubt" and "Service is life, life is service". Still she didn't hesitate to betray Apollyon to go to The Warden's side.