Katherine Campbell is one of the most iconic figures in the story of the Renfrewshire witch hunt. It was she who laid the curse of God on Christian Shaw, hoping that the devil would "harle her soul through hell." Campbell, in her early twenties, was an economic migrant, having left the Highlands a few years before in order to find work. She was hired by the Bargarran household at the end of May, 1696, as a domestic servant, and was only a couple of months into her employment before her troubles began.

The story goes that Christian Shaw caught the maid stealing milk, and told her mother, who in turn admonished Campbell for her theft. Tampering with milk, in whatever way, was often a common accusation against those accused of witchcraft, and it is perhaps telling that this crime lies at the centre of the story. Campbell went on to deliver her famous curse, unfortunately for her in front of other household servants. A couple of days later Agnes Naismith appeared at the house, and shortly after Christian Shaw's descent began. Christian Shaw soon cited Katherine Campbell, along with Agnes Naismith, as an invisible tormenter.

Campbell has been portrayed, in both oral and written sources, as a young woman with a spiteful nature and a short temper. Certainly, she would perhaps have been viewed as a cultural outsider, given that she came from the Highlands, and that she tended to drift between households. Christian Shaw claimed that the maid was one of those responsible for her fits and seizures, for the pinches, the scratches, and the slaps that she endured. The young girl claimed that Campbell was one of those responsible for making her dart about the room with unnatural speed, by carrying her with the other witches in an attempt to send her crashing against a wall, or to the floor. Witnesses, including local minister James Brisbane, testified that Christian Shaw named Campbell as one of her chief tormentors.

While Agnes Naismith prayed that the young girl would recover, it would seem that Campbell was unrepentant. When Campbell was arrested, a quantity of hair was said to have been removed from her pocket. After this, Christian Shaw no longer regurgitated any hair. It was small pieces of evidence such as this that were collected in order to create a case against the maid. On another occasion, Christian entered into a discourse with the maid, loaded with religious symbolism, in front of witnesses. She chided the invisible Campbell for her pact with the devil, and reminded her that there was no repentance in hell. Campbell was said to have been seen at meetings with the devil, where together they plotted the young girl's destruction, including drowning her in the well in Bargarran courtyard.

During her imprisonment, it is said that Campbell declared that "the doom pronounced on her was just, and she could not free herself of witchcraft." However, when she came to the point of confession, she experienced fits and convulsions not dissimilar to those suffered by Christian Shaw. This inability to confess was seen by some as evidence that her pact with the devil remained effective, given that her tongue was bound against confession. Campbell never did confess, and before she was executed, threw a curse upon the town similar to that of her alleged co-conspirator Agnes Naismith.