The Master

The Master is a renegade Time Lord from the planet Gallifrey, in the constellation of Kasterborous. When he and the Doctor, another Time Lord, were young, they were close friends. But, as he looked into the Time Vortex, he began to get mad. When the Doctor was with him, he managed to keep calm, but when his friend left Gallifrey, the madness intensified, making him choose to leave as well.

He has had nineteen different incarnations, at the end of which he died, sacrificing for the Doctor.