Lucy Saxon is the human wife of Harold Saxon, Or as he is better known as The Master, She is loyal to the master and stands by his side when he takes over Earth in the episodes Sound of the Drums and Last of the time lords.
She is the one in Last of the Time Lords who kills the master by shooting and him and he refuses to regenerate. She also appears in The End of Time part 1 & 2. She takes a forced part in resurrecting the Master. It is assumed she dies in the explosion when the Master is resurrected but there is no conformation of this.