Bill's mother at one point met the Twelfth Doctor, who would take many pictures of her during their brief time together. These would eventually be passed on to Bill.
Bill would often think of her mother, despite having never truly known her. She was often told that she looked like her, and she would imagine her mother telling her wise phrases despite her lack of presence.
Bill formed an imaginary version of her mother in her head that she would hold conversations with. She explained this to Nardole after he caught her having one of these conversations. When Bill attempted to stop the Monks invasion of Earth, she flooded their psychic link to the human race with memories of the imagined version of her mother. As these memories were pure and untainted by the Monks, they were unable to poison them. By using these memories, Bill ultimately broke the Monks control over Earth, forcing them to retreat from the planet. (TV: The Lie of the Land)