Seikoku no Ryuu to Powder Kiss

A boy named Kei Asagiri has part of his soul removed and sent to the Netherworld, granting him the ability to use powerful black flames but at the cost of his soul being gradually consumed. He works as an "Antique Hunter" for the Kaorun Company. After returning to Koarun Academy City in Japan, he is taken by an unusual group and brought to a distant part of the city. He is forced to become the owner of a rare antique in the form of a blonde British girl named Victoria. This antique is believed to be the key to saving Kei from his soul's consumption by providing excess power through daily kisses. This arrangement benefits both, as Kei releases excess power and Victoria maintains her human form. The challenge lies in the required method—kissing her once each day.