Originally from Hieronymous, Axton spent ten years with the Dahl military force, reaching the rank of sergeant. After his pursuit of personal glory and disregard for orders led to numerous compromised missions, his wife and commanding officer Sarah simultaneously divorced him and discharged him from the military. Sarah made it a point to highlight that this would lead to his death by firing squad, then ordered him not to flee to any of the numerous border worlds beyond Dahl's reach. Taking her not-so-subtle suggestion, Axton went AWOL, using his skills and turret as a mercenary on other planets. Axton became aware of the Vault thanks to a radio advertisement orchestrated by Handsome Jack, who had been monitoring the Commando's most recent bounty hunt. Although Axton was turning in more bounties and making far more money than anyone else in Sheriff Youngblood's jurisdiction, he found that it was too easy, and the lure of fame, fortune, and challenging combat drew him to Pandora.
As an infant, Maya was identified as a Siren and given to the Order of the Impending Storm, the ruling order of monks on her homeworld of Athenas. She trained her Siren powers in secret until she reached adulthood, when the monks revealed her to the public as their goddess and savior. Irritated by the Order's short leash, she longed for adventure and expressed interest in traveling to Pandora to learn more about her Siren lineage. She eventually realized that the Order was using her as a threat to extort money and obedience from the people of Athenas. After Phaselocking and executing her handler Brother Sophis, she headed to Pandora to finally satisfy her curiosity about Sirens.Maya already knew of the Vault and the presence of Eridium before arriving on Pandora. After Handsome Jack learned of Maya through Hyperion surveillance footage of Athenas, it presumably didn't take much effort to get the Siren on the train at the beginning of the game.
Aged 36, Salvador is a local, born and raised on the planet Pandora. He stands at a height of 5'4 (~163cm). This stunted growth is revealed to be due to heavy steroid use throughout his life.Despite his love for excessive violence being aimed towards bandits and criminals, Salvador was nonetheless about to be executed by the people of his own hometown of Ovejas for his actions when a Hyperion strike team arrived to seize the town. After destroying the invading forces, Salvador becomes interested in the new Vault upon hearing of its dangers while 'interrogating' the last survivor of the strike team and leaving him to crawl back to Hyperion on a single arm (as Salvador had ripped off his other arm and broken both his legs).
Zer0 is an assassin for hire. According to his wanted poster and his personal ECHO recordings, he performs both political assassinations and common hits. Following a particularly unsatisfying assassination where the target fails to fight back, despite being encouraged by Zer0 to do so, the assassin starts to look for a more challenging environment. He travels to Pandora after hearing of the Vault from a bartender, deeming the hunt as worthy of his skills. As his face and body are completely covered by his suit, a certain amount of mystery surrounds Zer0's true appearance. In an ECHO recording, Angel remarks that she's not even sure if Zer0 is human; both in-game and promotional material show that he has only four digits on each hand. His origins and real name are also unknown, leaving the character shrouded in mystery. As revealed in the 'Ask Dr. Tannis' Section of the Pandoran Gazette, included as a scrap of paper in the Diamond Plate Loot chest, Zer0 is not his true name. The remaining piece of scrap containing the name and gender of Zer0 were not included, but the part that does remain suggests that Patricia Tannis is aware of Zer0's origins.