Rust is a derelict MAD in the Equestrian Wasteland in the year 2728. He stands as the last fully-functional Mechanical Autonomous Drone and due to 200 years of isolation from CALVIN, his AI has evolved itself and he has gained sentience. His serial code is 845710-N.
Rust looks like a basic MAD though his black glossy armour has corroded and degraded, leaving him rusted over hence his name.
While other MADs remain active, they are highly rampant and savage, acting like animals while Rust retains high intelligence and has developed his own sentience. As such, he has feelings, opinions and a mind that can be confused, but he is still immune to Psionic attacks as he lacks an actual organic brain. He has also developed his own organic-sounding voice, as opposed to the processed and tinny voice previously given to MADs.
Rust was part of the drone forces attacking the Crystal Empire during Operation: Shattered Heart. In the battle, it sustained heavy damage and was left inoperable in a crater with Crystal shards from a Geode sticking out of it. Presumed destroyed, the drone was not recovered. The drone slowly repaired itself via the Lunarium present in it's plating and though it's own NanoBots. The Last Lights struck as the drone was mostly protected from the shockwave by the crater, offline and awaiting a Midnight official to revive it.
190 years later, a Scavenger recovered the drone and sold it to a mechanic in Bolt City. There, it was nearly fully repaired and functional but the drone remained dormant, still awaiting a Midnight official. It sat in the workshop for 10 more years, the mechanic unable to sell it, before Connor arrived and recognised the MAD. By touching it, Connor inadvertently awoke the unit from dormancy as it recognised his DNA pattern, showing him to be Apex Firebug. The drone explained to Connor that it was waiting for him and now was taking orders from him. The mechanic agreed to let Connor take the robot, having no use for it himself, and Connor named it Rust.
Rust continued to travel with Connor for the majority of the events of Fallout: Survivor, and appeared in Fallout: Counterpart as a cameo.
Rust suffers from "Repeating Receptor Data Intake" which is more commonly known as PTSD for machines. The problem occurs when a machine is left with a memory event considered traumatic and does not undergo the necessary processes to fix it. If the drone happens to undergo an event similar to what it had experienced to gain PTSD, it will trigger a strong (and often violent) response. In Rust's case, this involves randomly firing the Primary Laser Cannon during loud moments such as the heat of battle or even celebrations.
Rust's serial code, 845710-N, can be interpreted as "BASTION." This, along with his affection for the forest and wildlife as well as his PTSD, is a reference to the character from Overwatch known as Bastion. Both are the last of their kind in a line of advanced combat robotics, both with in-built weapons systems.