Kohaku's body clearly is unique even for "demon" standards, what Cleo thought should be pretty close to the truth of her physiology.
But still... the question is... how to feed her mana? If we take Cleo's own past "resurrection", this state Kohaku is in could take centuries to pass.
And considering her level as a caster, the mana necessary to "revive" her... could very much power up a country for centuries too.