Android 18

Android 18 (人造人間１８号, Jinzōningen Jū Hachi-go) is Dr. Gero's eighteenth android creation (although, she is technically a cyborg), designed to serve Gero's vendetta against Goku. While her interests do not deviate from this expectation, Android 18's curiosity to activate Android 16 in spite of Gero's orders not to do so lead Android 17 to take it upon himself to murder Gero (maybe from the fact that he kidnapped them while they were human in order to turn them into androids). She later becomes the wife of Krillin and the mother of their only daughter, Marron. She also unofficially joins the Z Fighters. Although she does not take part in any battles against Majin Buu, she fights Super 17 in Dragon Ball GT.

Android 18, like her brother, has shoulder length hair, but hers is blonde. She also has blue eyes, and loop earrings on both ears. She keeps the left side of her hair behind her ear. When she was first activated by Gero, she wore a blue jean jacket and skirt (the Red Ribbon logo was on the back of the jacket) with dark blue stockings, brown boots, and a black undershirt that had white and black striped sleeves. Later, she wore a black jacket with black gloves, a white undershirt, and blue jeans with a golden chain. When she was married to Krillin and living at Kame House, she wore a blue jeans jacket, white pants and black sandals. Later, at the World Tournament, she wore a black shirt, with black and white strips on the sleeves, in blue jeans. In Dragon Ball GT, she cut her hair shorter and wore business attire.
Initially cool and confident, 18's sarcastic humor later becomes the most striking trait of her personality. One example of her sarcasm is demonstrated when she is asked if her real name is legitimately "No. 18", to which she responds "My father was pretty dull" (in Japanese, she replies "That it doesn't matter" before tossing the number at the World Tournament Announcer). Shortly before this conversation at the 25th World Martial Arts Tournament, Goku takes notice of her assembly with the Z Fighters, leading to the retort "Look, he noticed. How cute" (in Japanese she says "Took you a while Idiot!"). Android 18 still exhibits some of her sensitivity, especially towards her twin brother. This is in stark contrast to her her alternate self from Future Trunks's timeline, where she is a murderous sociopath who cares for no one but herself and her brother

Her birth name is never revealed, and following her allegiance to the Z Fighters after the defeat of Super Perfect Cell, she is referred to as simply "18".

Information from:
http://dragonball.wikia.com/