Seikoku no Ryuu to Powder Kiss

Seikoku no Ryuu to Powder Kiss (聖黒の龍と火薬の儀式〈パウダーキス〉) is a light novel series by Akino Kitamoto with illustrations by Shirabi.

Plot
Kei Asagiri is boy who has had part of his soul ripped apart a sent into the Netherworld. This allows him to use the mysterious power of black flames stronger than any other magic, but in exchange for the lost part of his soul which is constantly being eaten by the denizens of the other side. Kei works as an "Antique Hunter" for the Kaorun Company. One day when returning from a job to Koarun Academy City in Japan, he was kidnapped by a weird but very capable group and brought to the far side of the city. By some weird but acceptable turn of events, he was made to become the owner of a very rare kind of antique in the form of a blonde British girl named Victoria. This particular antique is supposed to be the Key that can save Kei from being devoured by the other side by surpassing the magic power by supplying the vast and excess power to Victoria. It’s a win-win situation for both of them, since Kei gets his excess power out and Victoria gets enough mana to maintain her human form. But the only problem is the method―by kissing her once per day.