The Relic Eater contracted Kyouya Kirigaya and is designated as Malleus Malificarum V. Her intrinsic abilities pertain to the use of a wide variety of poisons both in and out of combat. Neros vibrant personality can be described as being similar to her abilities as her goading words and intentionally aggravating tone are venomous to her master. Like other Relic Eaters Nero seeks an attribute from her wielder and her chosen attribute is revenge. Because of this she must goad and evoke Kyouyas anguish and torment and breed it into anger and vengeance for her to be in top condition. For this reason alone she continues to mock and remind him of his shortcomings that had his squadmates killed and walk around in the guise of his dead friend. Outside however she still seems to be subservient to her master and while not hostile she was still prone to pick fights and argue with others. Source: Taimadou Gakuen 35 Wiki