Kuchikake Ryuu no Keiyakusha

Yao is a 16-year-old boy who inherited an antique shop left by his grandmother. As a result of being treated with care for many years, antiques that have developed a soul are known as "mononoka." When a mononoka's heart is shrouded in darkness, they become known as "koware." Yao's job as an exorcist is to cleanse them.

In order to awaken his older sister, Mei Rin, who was attacked by an "antique with a hideous soul" in the past and has stopped growing at the age of 12 and continues to sleep, he pursues the whereabouts of the koware who attacked her sister.

Just then, he receives an exorcism request. Eager to find a clue to awaken his sister, he is attacked by a koware right there and is driven to the brink of death. Despairing at the fact that he would die without having accomplished anything, he was saved by a giant dragon with ancient powers. What is this dragon's true identity and what was it that guided their meeting?