ニオ

A demon bound to the Supreme Archdemon's Tome. She can sense how powerful a demon is by sight alone. Her knowledge of demons is broad, Gil frequently turns to her when he's unsure of the nature of foe faced. Those who meet for the first time frequently mistake her for a fairy because of her appearance.

 Gil found the Supreme Archdemon's Tome in an abandoned shrine one day, and Nio has been contracted to him ever since. It's unknown what the terms of their contract is, but Nio lends her support to him unconditionally when it is time to exorcise demons. 

Although the forcefield created when Gil invokes the tome is strong, it can only be used after the target has been defeated in battle and is too weak to break free of its bindings. Before the tome can devour the demon, Gil has to break the demon's spirit and crush it utterly so that the tome can do its work. To that effect, Gil has figured out the fastest way to break a demon's spirit is to rape the vessel it inhabits. The combination of humiliation through sexual violation and any sexual pleasure unwantedly derived from the experience is sufficient to feed through to the demon residing within the host, rendering it vulnerable to the Tome's consumption of the demon from the host and its permanent banishment to the void. As Nio embodies the tome's power, she frequently comments on how tasty (or tasteless) the target demon is prior to its banishment.

Nio has been with Gil for so long that she's used to his raging libido and indiscriminate seduction of the opposite sex. She does take umbrage if Gil turns his sexual attentions towards her though.

When not doing "work", Nio spends her time chatting with Gil, his party members, eating and drinking her fill and generally having a good time at Gil's expense. 

Nio is contracted to both Gil and the Supreme Archdemon. In the event of conflicting orders between the two masters, it is the latter who takes precedence. When she binds Gil and prepares him for banishment, his decision determines whether or not she goes through with the Supreme Archdemon's will. If she does, she laments Gil's stubbornness and promptly vanishes together with the Tome, her purpose fulfilled. /