The Master's daughter
From Tardis Wiki, the free Doctor Who reference
At one point in his life, the Master may have had a daughter.
In her Missy incarnation, the Master began referring to her daughter and the day the Doctor gave the Master a dark star alloy brooch, but before she could elaborate, Missy was interrupted by an incoming Dalek. (TV: The Witch's Familiar)
However, in an earlier body the Master stated that he never had children. (AUDIO: And You Will Obey Me)
