The Author is the being who created the universe which the Crosslink Saga takes place in.
During the events of Chapter 0, Rimas Nolie attempts to send her memories eight years into the past. However, Praedi, the deity of knowledge, alters her magic. Three alterations were made to her magic. First, her body was sent into the past along with her mind. Second, she was sent to the beginning of the universe, rather than merely eight years into the past. Finally, Marcel Tano's heavily-injured body was also sent into the past.
As the two arrive at a point preceding their own creation, the author is then forced to come up with designs for them on the spot. Though annoyed at the intrusion, the Author decides to grant their wish of being delivered safely to their original destination in 540 GD if they can beat him in a fight. He gives the two their equipment of choice and lends them the power of creation so they can make their own weapons and training equipment.
The two then proceed to create the universe, starting with the sky and then Mersia itself.