The Himitsu Series is a trilogy of songs made by Hitoshizuku-P and Yama△. The series focuses on the forbidden love between the Angel Rin and the Human Miku Crynpto, who aren't allowed to be together because of their race and gender. Knowing this, Rin meets with a demon and exchanges one of her wings to become human. The two first songs in the series have been featured on Himitsu ～Mushoku no Chikai～ (秘蜜～無色の誓い～) and STORIA and a third song was released on Polkadodge.