Acid Mana is the main antagonist and the driving force behind the events of Kamisama no Iutoori and Kamisama no Iutoori 2.
Acid Mana is in fact Anam from 0-zero, a manga by the same author Kaneshiro Muneyuki and artist Fujimura Akeji.
She is an Earthling from the future, after the Earth has been swallowed up by the Sun. She, along with 99 others, won a lottery that let them board a spaceship to be the only people to escape the death of the Earth.
Acid Mana is over 400 million years old, and is the creator of the Earth that the main story takes place in. She created it, along with many other versions of Earth, so that her friend Mamoru could return to his family and so that she could ease her boredom.
She, along with Sein Kami, who she also created, enter the version of the Earth in the "most boring era" and grant Kamimaro his Tomfoolery, thus starting the entire story.