Sorry, my brain works a little wierd, I get a super basic concept: Droid that hacks and fixes stuff. Then I need to create the character sheet so I can see how that looks on paper, and what I can actually do. Then I can create the narrative for the character. In this case:
Rex (R3-X20) is an R3 series astromech droid who once belonged to a smuggler. Rex has had numerous after market upgrades and has not been wiped for a very, very long time. Working with a smuggler, it was important for Rex to be able to splice so they could avoid trouble (and cause a little from time to time) and work as a back-up pilot when things went sideways. He also needed to be able to repair the ship after their more dangerous jobs. With each upgrade, he became less and less like the base model and gained more personality quirks.
On a previous mission, Empire forces caught his companion off-guard and there was little that Rex could do to stop it. They killed his companion and confiscated all of his illicit goods, to include the highly modded R3 unit. R3's processes were disrupted by a restraining bolt and they would have wiped him, but they wanted to recover what information they could from him first... big mistake, they didn't realize he was equipped with a Data Breaker. He managed to disable the restraining bolt, steal a large swath of data, and send the few imperial forces into the vacuum of space.
He probably could have gotten away with it up to that point, there were no survivors and he could wipe the ships data, but a data loop kept reminding him the empire was responsible for his companions death. He couldn't help himself as he siphoned off their credits, his companion had always said the best way to hurt someone was by hitting their credit stick, and he needed to hurt the people who had ended his companion. His splicing didn't go undetected, and now Rex is known to the empire for the splice and the murder of the ships crew.
Realizing a lone droid was unlikely to survive very long with the Empire against it, he flew the freighter to the vicinity of a known rebel activity. The ship was shortly captured and the R3 unit offered his services to the rebels, it was also another way to hurt the Empire. The rebels too tried to wipe him, afraid it was an imperial ploy. One of their technicians wound up crumpled on the ground and their systems became flooded with the imperial data he had stolen, to include the imperial bounty on him. They seem to have come to a silent agreement of sorts, Rex will support the Rebels, but they aren't touching his memory core.