The Story of Twitch
Twitch was born in Zaun's sewers to a life that most would consider catastrophic, and he considers paradise. The plague rat has developed a genuine aesthetic philosophy around filth — he believes the alchemical runoff, discarded experiments, and glorious decomposition of Zaun's Undercity constitute a kind of beauty that cleaner people simply lack the sophistication to appreciate. His crossbow, modified with whatever he scrounges, delivers this philosophy as concentrated poison.
Over years of sewer-wandering, Twitch has synthesized an encyclopedic knowledge of toxins — which alchemical byproducts interact with which organic compounds to produce what kinds of systemic failure. He applies this knowledge the way other people apply artistic talent, mixing new poison formulas with the enthusiasm of someone who has absolutely no idea they're considered terrifying. He thinks he's an unappreciated artisan. He is not wrong about the "unappreciated" part.
Twitch's self-image is boundless. He considers himself a genius of "chem-tech gastronomy," a revolutionary thinker, and one of Zaun's great uncelebrated minds. He believes his contributions will one day be recognized. The residents of Zaun have a different assessment, but since Twitch is extremely hard to find when he doesn't want to be found and can kill from surprising distances, they mostly just avoid the sewers near his territory.