The Yin-Yang Spirit Series (陰陽 精霊 シリーズ, On'yō Seirei Shirīzu) is a series containing two exclusive Cursed Gears produced from the joint project between the Tōdō and Kuromatsu families as a sign of their long lasting kinship prior to their separation and eventual estrangement due to different philosophical views. The Cursed Gears, being the representatives of the Yin and Yang, are very powerful demons on par with the Black Demon Series, and therefore, requires a high level of demon resistance in order to wield one of them.
However, due to the series having only two available, it was deemed inevitable that the heirs of the Tōdō and Kuromatsu families are the ones to wield them since they were the creators of the demon series; the Tōdō's wielding Asahikaku and the Kuromatsu's wielding Yorutsukiryu, respectively.
There are two demons that are named so far.
Main article: Yorutsukiryu
A knife-type of Cursed Gear contracted to Spy Sen Kuromatsu. It is used to pierce and immobilize its target's pressure points, and it may manifest itself into a black dragon to deal a wide range of targets.
Main article: Asahikaku
A battle fan-type of Cursed Gear contracted to First Lieutenant Kotonoha Todo. It provides the ability to enthrall anyone weak from its powers and possesses them to commit suicide.