Chained Soldier, Vol. 5 (manga)

The 6th and 7th Squads team up to rescue the abducted Yuuki, kidnapped by a humanoid Shuuki...who turns out to be his long-lost sister Aoba. Upon learning about their situation, Yuuki tries to convince his sister to let go of her hostility toward the Demon Defense Force, but their conflicting ideals lead to a fierce battle between the two siblings!