If Jonathan Randall is Jamie's ultimate antagonist, Claire finds her match in Geillis Duncan. Geillis is the wife of the procurator fiscal (kind of like a judge) of nearby Cranesmuir village. She's also sarcastic, an herb gatherer, and presumed to be a witch who sells love potions and tinctures to help girls in "trouble" (in modern speak, she's basically giving them an herbal morning-after pill). Geillis is presumed dead after being burned at the stake, but not before Claire notices her smallpox vaccination scar, thereby revealing that she is a time traveler, too.
Until the whole witch-burning incident, in which Claire is almost a victim herself, she has a love-hate relationship with Geillis. Geillis is knowledgeable with herbs, but she's also a heartless gossip. When it is mentioned that witches are tested by pricking their fingers, Claire snipes, "I imagine they'd get ice water, not blood, when they tried it on you" (25.22). Ooh, burn… or brrr is perhaps more like it.
Geillis serves as a nice foil to Claire. She's basically Claire if Claire didn't have morals. She also shows us how women like Claire—think: knowledgeable, powerful—are treated in the late 18th century. Basically, they are feared and sometimes executed. Good times.
With Geillis however, folks almost have a point. This woman is keeping tons of secrets—she's been diverting her husband's money to France to assist the Jacobite uprising—plus she poisons the sucker. She's also been having an affair with Dougal Mackenzie and might have given birth to the heir of Clan Mackenzie had she not died. Yeah, this lady's definitely no saint.
Once we know that she's a time traveler, though, we find ourselves hung up on one question: Did she travel back in time by accident or is she seriously trying to change the future?