I've intentionally left some space before Anton came to Vyaki (playing off the whole memory-drain angle) for Cerebus to work with and 11 years of local history for me to fill in as it becomes relevant (and, well, because I haven't figured it out yet...)
Favored Enemy doesn't necessarily mean antogonism (well, I'm not saying it doesn't either but...

). What it does mean is that I've come across them often enough to know how they work, how to pick one of their agents out of a crowd, what questions to ask to track their movements within the city*, etc.
Wolfsbane is reported to be able to hold of werewolves,
holy water/symbols to repel undead,
some myths claim a demon can't cross a circle of salt,
fey can't stand the touch of cold iron.
As far as medicinal herbs go, foxglove (in very low doses) can be used to treat heart trouble, willow bark tea to treat a fever (naturally occuring aspirin), rosemary oil to treat headaches, and I think pomfrey can be used to reduce swelling.
That's about all the mythical-creature-repellants I can remember.
*On that note, Cerebus, since the Urban Ranger doesn't get Survival as a class skill, could I move that bonus to Gather Info instead so it works with Urban Tracking?