Barsac Senki: Katayoku no Rick to Shirogane no Rook

Rick Barsac is a hard-working child that despite her innate superior physical strength and hard training fails to unlock her talent as a spiritualist. The failure results in her being thrown away by her family.

Abandoned, the only ones that extended a hand to her were the demons, the greatest enemies of the human race. Serving under them as a soldier she becomes a weapon of human slaughter, a fiend that should never have existed.

(Source: Novel Updates)