Spiral: Bond of Reasoning

Ayumu Narumi's older brother Kiyotaka, a renowned detective and piano player, disappears all of a sudden. The only clue Narumi has, are the Blade Children. Two years later a row of murders and incidents begin, relating to the Blade Children. Together with school journalist, Hiyono Yuizaki, Narumi tries to figure out their destiny. (myanimelist.net)