Zerozaki Hitoshiki no Ningen Kankei: Zerozaki Soushiki to no Kankei

The Zerozaki Clan is a family of demonic killers, ranked third among the Killing Names. Having just become siblings, Zerozaki Hitoshiki and Mutou Iori attack Aikawa Jun—Humanity's Strongest Contractor—with the best intentions. But, as a result, get caught up in her work! They're headed towards the domain of the second-ranked Killing Names, the Yamaguchi Folks, on the island of Taiyakujima. Therein lies the Lifelong Undefeated Crystal Kaiser, Rikka Kajumaru. (Source: Kodansha, translated)

The Zerozaki Clan is a family of demonic killers, ranked third among the Killing Names. A boy with eyes like that of a dead man and a serial killer with a tattoo on his face. The story of their encounter and the never-before-told truth of the Kyoto serial killer case is finally revealed! The motivation of Zerozaki Hitoshiki, and its miserable ending......?! (Source: Kodansha, translated)