Michael Masterson (also called Mikey) was one of the children whom the Master encountered in Hexford and made his "children" by making them absorb some of his symbiotic nuclei.
Biography
Early life
Michael was born in 1968.
He had a brother who fought in the Falklands War, killing a man and becoming traumatized by it.
Meeting the Master
In 1984, when the bus driver that was carrying him, alongside Colin, Janine and Helen, became impatient of them, threw them out to the field. Michael convinced his friends that it was best to return home by walking than by bus, so the four of them got lost. They heard an explosion nearby, and went to the barn from which it had come from. The group heard as a man cried for help, and Mikey helped him, revealing he was the Master.
The Master hypnotized them, so they could help him recover from his decaying form, and assigned them to do several tasks for that purpose. The Time Lord started to see Michael as his favourite, and gave him a gun with which he had to kill a driver who could have endanger the operation. When Michael had done it, the Master's "spell" vanished, and he wasn't hypnotized anymore. (AUDIO: And You Will Obey Me)
Out of the Master's spell
Michael tried to reach his favourite teacher at school, Jane Hampden, but she wasn't at home. Instead, she was with Vislor Turlough and Tegan while the Doctor stayed at her house. As Mikey didn't want to tell him about the Master, he returned to the barn where he was expected.
Michael tried to get his friends out of the hypnosis, but he couldn't. Then, he attempted to kill the Master by damaging the machine he was in (with which the Master was trying to extract the symbiotic nucleus he had implanted to the four children to get his Tardis to pilot correctly). He thought he had killed him. (AUDIO: And You Will Obey Me)
Meeting the Doctor
Janine, Helen, Colin and him departed company. He placed the grandfather-clock TARDIS in the care of his brother. After said brother was murdered (actually by Annie of the Transhuman Sisters of the Unholy Protocol, but, Michael suspected, by the Master) in 2016, he set a trap for the Master by trying to sell it at an auction. After much happenings during which Michael was reunited with Colin, Janine and Helen, and the Transhuman Sisters and Dragonhunters both attempted to execute the trio due to the traces of the Master's essence still present inside them, it transpired that the Master's disembodie consciousness now resided inside his TARDIS's telepathic circuits.
Goading him into trying to destroy said circuits to permanently kill the Master, the Time Lord got him to touch the circuits and took over his body. Still in Masterson's form, he killed Annie before she could destroy Helen, Colin and Janine, and resumed his psychic control over them. Shortly thereafter, the strain of bearing the Master's essence caused the body to degenerate back into a copy of the Master's previous decayed Time Lord form. (AUDIO: And You Will Obey Me)