Rat King

The Rat King (鼠の王, Ratto Kingu, Nezumi no ō) is a mysterious being that began as a mouse that, at one point in its life, was given the ability to 'resurrect' a living being in exchange of his own life.

It is unknown if the Rat King has a legacy of its own or is related to the Undying King.

However, it is known that the Rat King hates tsunderes. The origins of this hate are unknown, but it has resulted in the Rat King devoting his life to the extermination of tsunderes.