TV-Series
Description
Hiro Hiyorimi is the protagonist of the story. He is introduced as a normal middle school boy who has just moved to a new city to live with his older sister, Sawawa. His ordinary life comes to a sudden and dramatic end when he sacrifices himself to save a stranger. In the anime adaptation, he pushes a gothic blonde girl out of the way of a falling iron I-beam and is killed instantly. In the original manga, his death is caused by being hit by a station wagon that went out of control.

Although he dies, Hiro does not stay dead. The girl he saved is actually a member of the royal family of the Monster Realm who goes by the name Hime, which means princess in Japanese. Using her power, she resurrects him, binding him to her service. His new existence is defined by a state referred to as half-immortality. This condition is sustained by Hime's blood, or in the anime, her flame of immortality. As long as Hime is alive and his connection to her power is maintained, Hiro will also live. However, this bond is a double-edged sword; if Hime were to die, he would die alongside her. Furthermore, if his flame of immortality is not regularly replenished, he will face a permanent death.

In terms of personality, Hiro initially finds himself thrust into a world of supernatural violence against his will. He is a reluctant participant in Hime's battles, which involve fighting off assassins sent by her siblings who are all vying for the throne of the Monster Realm. Despite his hesitation, he is not a coward. He demonstrates a consistent and growing bravery, largely acting as a human shield for Hime. His primary role in the story is to assist and protect her, utilizing his half-immortality to take blows that would be fatal for anyone else. He is described as not being very strong, making his willingness to throw himself into danger a testament to his character. Over time, as he learns more about Hime and her circumstances, his initial reluctance fades, and he becomes a more willing and dedicated protector.

Hiro's key relationships center on the inhabitants of Hime's mansion. His bond with Hime is the most significant; it evolves from a formal master-servant dynamic into something more personal. By the end of the anime, Hiro realizes he has developed romantic feelings for her and attempts to kiss her, an act that leads to an ambiguous reaction from Hime. He also works alongside other retainers of Hime, including the android Flandre, the half-werewolf Riza Wildman, and the vampire Reiri Kamura. Also living at the mansion is his older sister, Sawawa, who Hime hired as a maid. These relationships create a surrogate household around the princess, with Hiro often being the comic relief or the one most frequently killed in battle.

Hiro's character development is marked by his increasing acceptance of his role and his growing feelings for Hime. While he begins the series wishing to avoid the conflict, he ends it as a committed ally. His potential for growth is noted within the story. One character, Sherwood, states that Hiro has the potential to become the greatest blood warrior, though the specific reason for this is not fully revealed. In the manga, there are further hints about his significance, including a suggestion in chapter 35 that he has a special connection to the Monster World itself.

Hiro's most notable ability is his half-immortality. This power makes him incredibly resilient, as his wounds heal almost instantly and he is effectively indestructible. Even if he is killed, he can be revived again by Hime. This makes him uniquely suited to his role as her shield. Aside from this supernatural durability, he is portrayed as a normal boy with no other inherent combat skills. He is known to wield various improvised weapons from time to time, but he has no signature weapon or fighting style. In the manga, there is a detail that his body undergoes a physical change when Hime's life is in danger, a trait that was not adapted into the anime.