Note: the history below is inferred from the translated description of his backstory, and may not be entirely accurate.
The Frontier has long been a place seen by many as a land of opportunity, but also one of lawlessness. Many of the region's occpuants chose to engage in a life of piracy and other criminal activities, the Militia notwithstanding, making the Frontier a very dangerous place to live. The increasing adoption of Titans by these criminal elements, unable to be stopped through conventional force alone, led to the IMC's creation of the Mk.6 Automated Law Enforcement Officer (ALEO) - AL-6 for short.
AL-6's core is based on an amalgamation of much of the best combat AI data compiled by the IMC, based on Pilots and other AI units. Unlike many of the other robotic combatants employed by the IMC, his designers intended to create a guardian of the Frontier who can live comfortably with humans, as opposed to another simple mindless automaton. To that end, he is outfitted with a unique core designed to help learn 'human' attributes. AL-6's core programming is guided by one central law (a re-tread of Asimov's First Law of Robotics): "Robots do not tolerate human dangers."
AL-6 is one of the survivors of the infamous Battle of Demeter, during which he piloted a Destroyer-class Titan named "Curtana". Following the battle, he turned towards bounty hunting and peacekeeping, exceeding expectations in the tracking, apprehension and - if necessary - destruction of outlaws and criminals.
At some point, AL-6 was employed by TALOS to help pacify any perceived enemy forces present on the IMC planet Tristan. Once there, the now-renegade AI issued its true directive to kill all humans, attempting to exert control on the mechanoid Pilot as it had most all other robots on the planet. To the killer AI's surprise and dismay, Preacher refused or was otherwise unable to comply with the new orders due to his programming and core directive. This led to AL-6's departure from the IMC.
To this day, Preacher is an independent Pilot, roaming the Frontier as a defender of those he sees fit to protect. He will go to extreme ends to hunt down "dangerous forces" on the Frontier, and his pursuit to dispense order and justice is never-ending.