He is her successor. She gave him her powers when she saw him protecting both her and his sister. She loves and cares for kojou greatly and viceversa she later becomes his wife.
Vatler had proclaimed a few times that he was in love with the previous Fourth Primogenitor, more precisely the 12th familiar's master, Avrora Florestina. However, the exact nature of the feeling or the full extent of it has not been explored further, but during vol 4 its show he possesing a defiant look when facing avrovra in Nagisa body not show nothing of the so call love, that probaly being a lie he use as excuse to get close of Kojou .
Avrora's consciousness currently resides within her she considers her to a sister being worried over one anothers well being.