The gatekeepers primary tool are seals they have gathered to summon at will.
A seal is a card which houses a fragment of a soul of another being, this soul is called a subject as they are subservient to whoever activates the seal. The stronger the fragment the stronger the seal must be. The stronger the seal the more Will the user must imbue to activate it.
When a seal is activated the housed soul seeps into the users own, syncing with it, and allowing the host to temporarily access the souls powers as if it were originally their own. The host gains the attributes and abilities of whatever the subject has and how they use it depends on the hosts ingenuity before it dissipates.
Some subjects require special requirements be fulfilled before they allow their seal to be activated so a Gatekeeper must keep this in mind before adding it to their arsenal.
Each Seal leaks out a portion of their fragments spirit passively and must be recharged with the original soul periodically. A gatekeeper can only handle so many cards at a time and if they exceed their limit those cards cannot be activated.