Japanese Imperial Demon Army
Yuichiro Hyakuya
A young soldier in the Moon Demon Company who has been experimented on by the humans, leading to a part of Yu not being human as Asuramaru revealed. Asuramaru becomes his demon weapon after Yu refuses to give into him. He haunts Yu in his dreams for the first three months after they form a contract. When Yu took his training to the next step but asked to be Asuramaru's friend, the demon eventually gave in again and stopped giving him nightmares. Asuramaru has no desire to reveal any secrets or truths to Yu, and seeks to take over his body.
Guren Ichinose
Commander of the Moon Demon Company and one of the developers of demon weapons. Mahiru Hiragi leaves Asuramaru in his care, and he in turn gives Asuramaru to the Hiragi for research purposes.
Vampires
Mahiru Hiragi
Asuramaru's former master. Guren Ichinose's fiancée and lover.
Mikaela Hyakuya
A young vampire. While battling Yu with nightmares, Asuramaru often takes the form of Mika when he was 12 years old. Asuramaru encourages Yu to escape with him before Yu loses his humanity altogether. Yu's childhood friend.
Krul Tepes
It's revealed in the light novel that Asuramaru was known as Asura Tepes in life, and Krul inquired Mahiru Hiragi of her lost relative when she confronted her. He may still remember her as Yu notes he thought he felt Asuramaru's emotions when he made eye contact with Krul.
The First Progenitor
A mysterious and the first vampire who convinced him to turn him into a demon and renamed him as Asuramaru. He seems to be fond of the first progenitor as he addresses him as 'father'. He was the only vampire that the first progenitor was interested in.