Boukyaku no Senritsu  

bns, Forgotten Melody, La Melodía del olvido, moo, The Melody of Oblivion, 망각의 선율, 忘却の旋律

In the 20th century, there was a war between humans and monsters. Afterwards, the monsters took over the world but kept themselves hidden from public view. A young boy named Bokka is curious about what happened to the Meros Warriors who defended humanity against demons during that time. He meets Kurofune and learns of the power of the Meros and their I-bar machines used in battle. The warriors are the only ones capable of hearing and seeing the Melody of Oblivion, a phantom girl hidden away waiting to be rescued and become the savior of mankind. During a fight between Kurofune and a demon, Bokka discovers something incredible - he can also hear that melody. As he continues on his journey, Bokka encounters various monsters, their agents, friends, and companions while learning about the full extent of his new abilities. He must battle evil in order to release Boukyaku no Senritsu and restore a world that has forgotten its beautiful melody.