From SuspireWiki

A Guardian is a Kindred tasked by the Ordo Dracul with guarding something important, be it the Chapter House, a Wyrm's Nest, some occult artifact, or even the secrets of the covenant.

Unlike the Sworn of the Axe that serve as the overt military of the Ordo, the Guardians are more of a "secret police" force that deals with the protection of important things (be they locations, knowledge, people, or something else) and counter-espionage.

The Kogaion is the pre-eminent Guardian in the city.