Io

Io is a mastermind of manipulating the dead. She is nice in nature, but her actions indirectly caused a huge mess against Meruia due to the undead being alive. This lead to the King of Meruia being charmed by a succubus, which led to more chaotic events. She is the only Majin that never fought against Celica, and helped him in the process when his lifeforce was drained in the later chapters.