A Guardian was an Osiran service robot encountered by Bernice Summerfield in a pyramid on Mars. The one that followed Benny around always lied whilst the one that always told the truth was destroyed by an explosion as a result of the war raging on the planet surface. It used the voice of Benny's deceased husband, Jason Kane.
Eventually, the Guardian told Benny to press the button that allowed Sutekh to gain a physical form and escape Mars as it deceived her about the nature of the control panel she was operating. After the Seventh Doctor was seemingly reduced to dust it lied and told her he was not really dead before walking off and leaving Benny alone. (AUDIO: The Pyramid of Sutekh)