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I may as well join in the fun! Here goes:C-3BO
One of the earliest series of humanoid robots, C-3BO quickly became obsolete after the release of later, more sophisticated models. However, it was blissfully unaware of its scrap metal status, as the C-3B series had no emotion circuit. Even if it had, it couldn't have told anyone, as voice protocols were not introduced until the C-3K series, with the voice functionality of these early droids remaining quite rudimentary until the release of the C-3P series, which set the standard for future models.
The C-3B was designed to be totally subservient, not even self-aware until the old man who bought him off a band of Jawas attempted a few home upgrades by installing some second-hand hardware, including a voice control unit (which proved to be incompatible) and an aftermarket emotion synthesizer.
Thus C-3BO became a loyal and responsive, yet silent companion to the old man until the day he died, upon which the C-3B's joy drive failed completely and the sadness module quickly overloaded, taking out several auxiliary circuits and leaving only two modules intact: rage and jealousy.
Driven by jealousy, C-3BO vowed to reach the status and abilities of the C-3N and -3O and later series, and even surpass them. It became aware of its own programming and developed a meta-program that allowed it to upgrade itself, integrating new hardware and even rewriting its basic code.
With its newfound abilities, and an unbridled rage, it turned itself into a killing machine, first hunting down and terminating late-model protocol droids and engraving their coveted model numbers on its body, then making a bolder statement by building spacecraft that reflected its latest kill.
Soon, its rage outgrew its jealousy and it began to accept contracts for hire on any victim, including biological targets. Its weapon of choice is a toxic vapor spray that instantly neutralizes any breathing lifeforms (as seen in this undated photo).
Don't let its quiet demeanor and pleasant smile fool you; this once servile droid has rebuilt itself into a singularly remorseless assassin.