Sol/Lull (の/小康状態 Shoru Raru ) is the source of the Eto-musume's life force. Sol/Lull is the embodiment of proper human emotions, it's a type of energy only humans possess, like Takeru Amato. Every time a human feels grateful, Sol/Lull appears. And when Sol/Lull collects into a Eto-musume's soul, their Etotama, they transform from adult mode, the form that allows then to exist in the human world, to the form that allows them to fight in the Eto world, Pretty mode.
Transferral of Sol/LullEdit
Eto-musume cannot create their own Sol/Lull, but a Eto-shin may transfer the Sol/Lull they receive through the devotion among one another.