Bettenou is a god (according to Inga, she is like a deity) with the power to turn people's words into a semblance of reality. ~!However, it is revealed by Shinjuurou that she is a misbegotten soul of mere words. Her original form is similar to that of a large, white-coloured rabbit with pale eyes and markings on each eye.!~

(Source: Un-Go Wikia)

He pursues his job as the "Last Great Detective". Others call him the "Defeated Detective". What's certain is that he's Shinjurou Yuuki, a young man with a passion for mysteries and a talent for solving them that has made him the target of dark forces now stirring within the sinister underworld of a near future Tokyo. That could prove lethal, given that not even the other "good guys", including the police, are exactly on Shinjuro's side. Fortunately Yuuki's not completely on his own, and with the aid of his uniquely talented associate Inga, he's ready to cut a swath through the veils of secrecy that have been laid before him. 
 
Get ready for mind against matter and a lot of cloak and dagger as the ultimate battle of clue and deduction begins in UN-GO! 
(Source: Sentai Filmworks)

This prequel takes place at the same time as the first episode of the TV series. As the story begins to unfold, more mysteries that were unanswered in the first eleven episodes come to light, and we learn the full story of the duo’s story. (Source: HIDIVE)