18 is, as her name implies, the eighteenth in a set of twenty one artificial humans created by Dr. Gero (later known as Cyborg 20) to aid the Red Ribbon Army in combat, and later to avenge their destruction by killing Son Goku. After being punched out of Cell by Gohan, however, she decides to join the Z-Fighters and even ends up marrying Krillin. The purpose of this marriage was presumably to piss off her dead parents, without having to marry a black guy.
18's original name was Lazuli, though dialogue in the anime and its lack of mention in the Manga implies that she has either forgotten or willfully abandoned her own name. This may mean she is prideful of her cyborg body, or that she simply has a reason not to think about her old life.
18 is the first character in Dragon Ball canon to defeat a Super Saiyan on her own, during her fight with Vegeta. While the two start the fight at similar strength levels, 18's unlimited power supply easily outlasted Vegeta's new super saiyan form, weakening him and soon after effortlessly incapacitating him. With training, she eventually became powerful enough to stand up to the likes of Hercule Satan, the world's greatest fighter.
18 and Krillin have a daughter together, named Marron (Based off of a japanese word for chestnut, like her father's). The fact that she shares a name with Krillin's filler girlfriend is best left undiscussed by those who prefer their humeri in one piece.
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Krillin and 18 are dirt poor, jobless bums, living in Kame House and rendering Roshi's "turtle hermit" title meaningless, since he now lives with enough people to make a whole damn town. 18 is often the one bringing in money, as seen when she blackmails Hercule into giving her the prize money for a tournament in exchange for letting him win.
Fact: Cyborg 18 is the second best Dragon Ball Girl
