All children of fire, both fallen and divine, have certain affectations. They carry these into the material world. When angels take human guise (the Hidden Form), this signature manifests itself. Each character has commonality that runs through all human forms he takes. For instance, an angel may only manifest in the form of children or perhaps the human form they take must always wear some piece of green clothing. It could be the angel only takes the forms of people with certain afflictions. It might even be that a certain symbol is always on the manifestation. There are endless possibilities, and all ideas should be discussed with the storyteller. Whatever the player chooses, it should be distinctive and add to the overall concept of the character.