The Master is a fictional male Timelord in the Doctor Who series. The only other Timelord still around, whilst the Doctor works for good, the Master causes mayhem. In the early series, the Doctor always looked the same, he had dark hair and a goatee beard. With the revisioning, he like the Doctor now has a new face. He was as it has been hinted created to combat the Timelord's enemies.
When Peter Capaldi became the Doctor, they shook up the Master's character. Instead of having as a man, they created his as female instead. The Master had appeared during Tennants episodes as a man, first played by Derek Jacobi and then by John Simms. The change to being a female was liked and hated in differing measures.