Bertha (whose surname we never learn) is one of the many minor characters that we only ever meet third-hand. She was a servant in Randolph Henry Ash's household, and her role in Possession is a little bit enigmatic and strange.
As Ellen Ash's journals record, during her husband's trip to North Yorkshire in the summer of 1859, Ellen suddenly realized that Bertha was pregnant. After discussing the matter with her sister, Patience, and a local reverend, Herbert Baulk, Ellen decides to send Bertha away to a Magdalen Home—a dreary institution where women in Bertha's position could work for room and board. But, before Ellen can follow through with her plan, Bertha disappears.
Being the suspicious twenty-first century readers that we know you are, some of you might wonder if you're supposed to suspect that Randolph is the father of Bertha's child. For our part, we think it's unlikely: Possession is way too earnest about Randolph Henry Ash's goodness and purity of heart for us to believe that he would take advantage of any woman, let alone a member of his household.
Instead, we propose that Bertha's role in Possession is to foreshadow Christabel LaMotte's own disappearance when she realizes that she's pregnant. In the mid-nineteenth century, there were few social support systems for single women who bore children out of wedlock. For some, disappearing into the night would have been one of very few viable options.