Akuma is an eye in a tome that speaks with a voice like sand or dust falling to the ground. He is Seth Mase’s familiar and describes himself as “a creature of law” – though Seth rather suspects that his true nature is not quite so kindly. Akuma has promised to grant his knowledge to Seth, but in exchange Seth must allow Akuma to accompany him on his adventures and speak freely. Seth consults Akuma for information about the planes, their denizens, and, occasionally, advice.
When Akuma appeared in the Material Plane, he brought his tome with him and became magically bound to it in a way he himself is hard-pressed to explain. Akuma knows when anything is written in this book. Seth has chosen to use this book as a one-way messenger to allow the citizens of Adastra to communicate with him. Akuma relays these letters from Adastra to Seth as they are received.
Mechanically, Akuma is a book imp, as described in Arcane Power. He grants Seth +2 to Arcana checks and the power to speak Infernal. While Akuma is in his active form, an elderly bespectacled man with wings and a lot of pointy bits, Seth can make him invisible as a minor action; Akuma is a perfect spy.