He hadn't meant to find her. He had just acquired his TARDIS back from the end of the Universe, because he certainly couldn't grapple with the Doctor without one. He settled for randomly flying around, visiting old battle sites and formulating a plan in his head. It was on some tiny little planet he visited on a whim when he found the Doctor's daughter.
She ran into him as she turned a corner, falling to the ground. The Master had thought she was a minor inconvenience until she jumped up, grabbed his hand, and yelled "Run!" He knew, he knew, who she was at that moment. Her hand was exactly the same, the mirror image of the Doctor's first incarnation. He had smiled, now knowing exactly how he could defeat the Doctor.
"I know your father," he had said after he helped her save the planet and she told him, so blindly naive, that she wasn't human. "Would you like to go look for him?"
Her name was Jenny, she was his clone, she died on Messaline and came back to life, but he left before her regeneration could happen. The Master smiled and nodded as she told her story, not believing his luck. The Doctor didn't even know she was still alive and now she was all his.
He helped her save planets, slowly trying to taint her. Every time, the solution was deadlier. At first she dug in her heels, repeating the Doctor's quaint little morality lessons. But the Doctor had only known her for a day, and he was never as good at manipulation as the Master. Eventually he got her away from saving planets at all, just traveling and seeing the stars, stepping in only when their own lives were in danger. Eventually they would start problems before they walked away, laughing at the inhabitant's stupidity as they destroyed themselves.
And Jenny went along willingly, never questioning him.
The first time he seduced her they had just escaped from a World War I battle, racing ahead of a cloud of toxic gas aimed at the trenches. He wanted her to see the humans the Doctor loved so much at their finest, killing each other off in horrible ways. She had cried and he comforted her before he kissed her, and she followed eagerly. She was so much like him, the way she arched her back and the little noises she made. The breathy babbling was even there, although she usually stuck to point and didn't start rambling about philosophy as he was known to. And the first time she used her tongue, the Master was certain that particular trick was genetic.
Eventually he got her out of her tee shirt and pants, opting for high heels and a form-fitting dress. Her hair curled around her shoulders and she was really quite lovely, something the Master didn't mind holding out on his arm. When the Doctor saw them it had been an accident. They were stopping to get her more clothes and she walked back to the TARDIS, modeling one of the dresses. It was short, showing off her long legs and slender arms. The Master had reached out to her when suddenly the Doctor's presence filtered into his mind, and he looked across the crowded square to see the Doctor stepping out of his own TARDIS with one of his silly human collectibles.
The Doctor's head shot up as the Master registered on his consciousness, their eyes locking across the crowded square. And then the Master kissed Jenny long and hard with his usual amount of possessiveness and she kissed back the way he taught her like the good little girl she was. He held the door to his TARDIS open and she stepped inside, not realizing her father was watching her, his face torn between fury, disbelief, and confusion. The Master turned to him and waved as he stormed forward, mouthing two words before he stepped back into the TARDIS to take Jenny away again.
"I win."