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 Bedford Station recording, a 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.
A9-51 is mentioned only in Fallout 4.