A Remix Heart is the perfect fusion of a Grimoire and a person. The only known Remix Heart is Mai Natsume who perfectly merged with the No-Name Grimoire. It is known that a human can fulfil the role of the person, but it is unknown if other species can.
Remix Hearts are exceptionally rare. Even Nine the Phantom, the creator of Ars Magus, did not expect to see one in a working form. [1] A Remix Heart, if fed on, allows one to become a "body of Azure" [2], and it is also necessary to create the Burning Red Grimoire. [3]