Pops, a T-800 sent back to look after Sarah Connor as a child.
As the years passed, Guardians’s skin covering aged like a normal human’s and his robotic body was somewhat in a state of disrepair in the present day.
While Arnold Schwarzenegger’s age wasn’t an issue in the first three Terminator movies (though his skin was sagging a little in Rise of the Machines), it definitely needed to be acknowledged by the time Terminator Genisys rolled around.
That movie had Schwarzenegger playing Guardian, a.k.a.