Penny (Shadow World)

Penny was a student, who was really a Shadow Person existing in a simulated world created by the Monks.

history
She went on a date with Bill Potts, and also met Bill's foster mother. She stayed at Bill's flat, and talked with her about trying to define their relationship. Bill told her that it was nothing to worry about. She then proceeded to encounter the Pope, which freaked her out causing her to leave. The actual Twelfth Doctor later told Bill to talk to Penny in the real world. (TV: Extremis)