A second self.
Many can't spell 'alter ego'. I've seen it in print as 'older ego' and even 'alter eagle'.
The uses of this phrase, which derives from the Latin 'other I', range from psychoanalysis to films and popular fiction. The term is also the name of a popular computer game. Recently it has been much used in stories of superheroes like Superman and Batman, who require a day-to-day human guise. Before that we had Dr. Jekyll and Mr. Hyde.
The concept of an alter ego, that is, a second self, was put forward by Cicero as a philosophical construct in 1st century BC Rome.
The term is first recorded in English in 1537, in a letter written by Richard Layton to Thomas Cromwell, regarding the suppression of the monasteries:
"Ye muste have suche as ye may trust evyn as well as your owne self, wiche muste be unto yowe as alter ego."