The Master's daughter

At one point in his life, the Master had a daughter.

Biography
For a time, the Master shared a home with his daughter. He never allowed her to have pets, as he felt that "once they outgrow the house, you just have to put them down." (PROSE: The Liar, the Glitch and the War Zone)

On Skaro, began an anecdote involving her daughter and the day the First 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)

Behind the scenes
When writing The Pirate Planet, Douglas Adams considered that the villain could be the Master's daughter, who was collecting the planets where he'd been defeated because she either loved him or hated him. (PROSE: The Pirate Planet)