Hiromi Kaname is Mirai Kaname's grandfather and a scientist specializing in research on the demon realm, called the Expanse or Makai, and its impact on the human world. After a rock rain destroys Setsuna Kai's apartment, Hiromi approaches him. Using specialized equipment, Hiromi identifies Setsuna as a Devil Child carrying demon DNA known as the DeviGenome, enabling travel between the human world and the demon realm.

Hiromi takes Setsuna to his laboratory-residence. There, he explains that Lucifer, the Demon Lord, seeks to conquer the Expanse by magically branding and corrupting demons into evil entities. These corrupted demons cause disturbances in the Expanse that manifest as bizarre phenomena in the human world. Hiromi reveals only Devil Children can enter the Expanse to confront these demons. He tasks Setsuna and his granddaughter Mirai, both Devil Children, with stopping Lucifer's plans to protect both worlds.

Hiromi aids Mirai in her investigation of the Expanse to help locate her missing father. His laboratory serves as a base for Setsuna and Mirai during their interdimensional missions, where they utilize devices called Devilisers and partner with demon companions.