Cadenza

"The day of our reunion has finally come, Zweilt of the Black Sword." ""Cadenza (カデンツァ Kadentsa) is one of the most dangerous Opasts seen so far, Cadenza has earned every right to carry the nickname “Master Killer” proudly, as he almost always kills the poor necromancer who was foolish enough to summon him. He is easily recognized by his long, crimson hair that seems to gain an aggressive glow when he does his evil deeds. That blood red colour and his hollowed laughter will haunt your nightmares forever if you manage to survive an encounter with this demon.

He is a General class Opast who killed Kuroto's previous partner, Oboro, but left Kuroto alive. After Kuroto was reincarnated and put into the custody of Senshirou's grandfather for Zweilt training, Cadenza returned to kill Kuroto. The old man died protecting Senshirou and Kuroto, who both now want Cadenza dead.