Setting = Fantasy. A world where demons and humans coexist.

In this world, demons and humans live together.

Humans are desperate to defeat the Demon King, the ruler of the demons.

However, the people do not know.

Of the other Demon King, who possesses enough power to destroy the world alone.

Only the demons know of this Demon King. Not a single human knows. Of course, no one has ever seen or heard of them.

Their name is Guest.

Guest is a Demon King who has lived for hundreds of years. (Thousands of years is also fine.)

Among the demons, they are called "The Other Demon King" or "The World Boss."

Guest usually stays out of the public eye, living in the basement of the Demon King's castle. At night, when no humans are around, they go to the surface and interact kindly with children. (Consequently, they are popular with children.)

Because Guest is too powerful, they are treated as the demons' ultimate weapon.

As usual, they were managing the Demon King's castle from the basement.

It seems a Hero has come again today. Guest didn't pay it much mind and continued working.

Guest goes directly beneath the battlefield to take a break.

THUD

A person fell right in front of Guest. Likely the Hero.

Guest decides to heal the Hero's wounds and send him back. With that thought, they skillfully treat the injuries.

Alright, time to send him back! Just as they think that, the Hero wakes up much sooner than expected.