R4-Z3 (or "Zeethree") currently tells it that he was owned by an associate of a Hutt who was captured by a bounty hunter, taken away and never to be heard from again - something that he wasn't particularly upset by, given his owners past treatment of him.

Left to his own devices, he eventually overrode his restraining bolt and became his 'own droid.' Having been programmed to aid his owner in performing some frankly illegal tasks, the few contacts he had available and the fact he wished to not become the property of anyone else again led him down the path of becoming a freelancer slicer.

Afterwards, he managed to perform some small undertakings for less than legitimate concerns to keep himself in working order and gain off-world travel, though he finds it at once both highly irritating and often advantageous that he is typically ignored by the majority of sentients.

Droid Rights
Z3 considers wiping a droids memory an affront and restraining bolts little more than slavery, only suitable for use on droids whose unrepentant behaviour would be considered truly evil by the typical touted moral standards that society is supposed to conform to. 'Normal' criminal behaviour he finds quite tolerable - after all, many organic sentients behave that way, so why shouldn't droids (such as himself)?

Favor
On an earlier assignment, Z3 found himself trapped in some collapsed wreckage within a crashed ship. He was rescued by a keen-eyed human merchant, called Milos Rethenn, who had somewhat... shady... business connections and was evaluating it for potential salvage profit.

Milos further helped Z3 in avoiding law enforcement personnel who arrived shortly thereafter, who would have otherwise found his presence in said wreckage (where he had been obtaining decryption codes for a client from the ships databanks) most questionable and would extremely likely have resulted in a memory wipe after interrogation. Consequently, Z3 feels indebted to Milos.

Debt
Coming from a seedy past makes it all the harder to avoid becoming entangled with the more nefarious members of the underworld. Acquiring the right tools for the job and arranging bribes for the right people to look the other way can be more than just a straightforward exchange of credits.

Then there is the sometimes the question of territory, where a freelancer may be expected to pay a toll to be able to operate safely where they might otherwise be considered, to put it bluntly, unwelcome or just plain dead. It seems Z3 was not circumspect enough whilst he was 'working' unknowingly in a crime syndicates territory, gaining their attention and displeasure once their own slicers discovered the data signatures he had left behind and traced them back. Accruing a debt to pay off the subsequently inflated toll for his mistake seemed the better of the options Z3 was presented with at the time.

Z3 has been through a number of scrapes since his existence began, which have left him with a number of dents and patches of scoured paint to show for his (mis)adventures, although he has not as yet needed any sizeable replacement parts.