So one of my first ideas that has stuck all the way through was the idea that each of the Demi-Angels has been the idea that they have the sigils of their fathers on their inner wrists much like tattoos but they are more like birthmarks. (fig 1.)
No mortal can see them so they don't have to worry about people asking questions, but even if they could they would just be able to explain them away as tattoos. To give you an idea of what I am talking about the sigils of the Big Four are below. Ariel is Uriel it just depends on the translation and that one is green, Raphael's is gold, Gabriel's is Blue and Michael's is red.
This books means so much to me that I even got Uriel's sigil tattooed on my foot (fig 2.). I did base Marissa off myself, Uriel is the angel I have looked to in the past who has gotten me through a lot so this tattoo is special. It was also my first one and before anyone asks the reason I got it on my foot instead of my wrist was because Uriel is associated with the Earth chakra which is located three feet below your feet and I couldn't get a tattoo there so I got the next best thing!