A9-51 was a synth who prior to 2287 escaped the Institute with the help of The Railroad and a Railroad agent named Dutchman. He only exists in-game as a voice on the holotape found at Bedford Station in the terminal inside the watchtower.
If generated in game, A9-51 is a Gen 2 synth wearing an Institute uniform. The voice on the holotape does not match this, being that of a Gen 3 synth (i.e. human).
A9-51 is only mentioned in Fallout 4.