Jadis

From Shadow Unit Wiki

Jadis is a nickname for Madeline Frost, presumably because her personality reminds people of Jadis, the White Witch of C. S. Lewis' Chronicles of Narnia.

Chronologically, the first reference to Frost as "Jadis" appears to be the following entry in Chaz Villette's LiveJournal by Daphne Worth:

I was this far from lying down on the spare autopsy table in the morgue, but Duke was there and I was afraid the pictures would get back to the kitchenette. Or Jadis. Whatever her fantasies are, I don't want to feed them. [1]