Cores are artifacts sent through the earth by Michael, Uriel, and Gabriel in Raphael's memory so his reincarnation could find them later on. There are eleven existing cores and one artificial core.
They are used by the user to invoke their fragment into an armor that acts as a second skin. The Gates of Argus in the front unleashs the power by opening, eleven wings coming out and going into the holder, invoking the fragment directly.
While the appearance of each core is different from each other, there are two things each cores has, The cross as the basis of the core, and the Gates of Argus in the front.