Fujiwara no Mokou is an immortal human. Immortality is coveted by those in power and is said to be an incredible ability. In reality, however, it is more akin to a curse. Losing the ability to die entails the dissolution of the boundary between life and death. Just as there can be no shadows without light, in the absence of death, life loses all meaning. Mokou is neither alive nor dead while also being both.

Keine Kamishirasawa is a were-hakutaku with the ability to edit history. Ordinarily, she teaches at the village school, but when there's a full Moon, she transforms into a hakutaku. She was originally a regular human but due to an incident in her past, she became the way she is now. When a human attains the power of a youkai, it tends to destroy them, but it seems Keine has remained an ally to humans and uses her power to help them.

Mokou and Keine trusted each other completely. Perhaps the fact that they were both once human contributed to this. They didn't avert their eyes from each other's past crimes or pretend they didn't exist. They stood together, having accepted everything that occurred on the road to this moment. Each existed―you could even say lived―in the other's heart.

A stifling wind of ash gray blew. Their enemies were close now. This incident was unlike any other. The intentions driving it were unimaginably evil. Coming face to face with it, Mokou and Keine couldn't help but picture impossibly dark futures for each other. If the immortal died and the historian lost her memories, each would be left with disfigured holes in their hearts, as if they had been gnawed by a horde of insects. They prepared themselves for battle. Their bond couldn't be stronger, and they trusted one another to watch their backs.

"Don't try and protect me all alone," said Keine, grabbing hold of Mokou's hand. But long ago, when the shrine maiden and her friends attempted a test of courage, it was Keine who ran off ahead, saying she'd protect Mokou. "That's my line," quipped Mokou, making no effort to free her hand from Keine's. Mokou was putting up a brave front, but there was a fire of desperation burning in her eyes. They lived in each other's hearts, which meant any pain they suffered was felt by both of them. "I'll protect you," they said in perfect synchronization as they clenched their fists and welcomed their foes.