Beyond the Stars
Old Gran to the Firman Boys
Mercia is in truth some sort of great aunt to the Firmans, her sister having married into the family many generations ago. She has always been a source of wisdom, and was the only one not to completely abandon Douglas when his time with a cult came to light, instead seeing it as an opportunity to set him on the straight and narrow.
With a keen sense of when things are about to go terribly wrong, she tried to instruct him in the ways of herbalism and salves, as she was trained in her youth, but in this regard he turned out to be somewhat terrible.