Character Description
She was a human who was cybernetically altered into a cyborg by Dr. Gero. She is the twin sister of Android 17, and later becomes the wife of Krillin and mother to their daughter, Marron. Her original objective is to kill Goku, she was programmed to do so by her creator Dr. Gero, in retaliation for Goku's destruction of the Red Ribbon Army. She eventually winds up being absorbed by another one of Gero's creations, Cell. During the Cell Games, Cell is forced to vomit #18 back out thanks to a crippling blow from Super Saiyan 2 Gohan. She is later seen in the Majin Buu Saga's, where she is married with Krillin, and has also given birth to a daugher. She is mainly featured as a supportive character and except her fights at the Tenkaichi Budokai, hardly does anything noteworthy. In Dragonball GT, #18 resumes her roll as a supportive character. Her main involvement with GT's plot, is her aiding Goku to victory against Super Android 17.
18 also appears in an alternate time line. In that time line, she and #17 wreak havoc, kill most of the Earth's populace and most of the Z-fighters as well, except Trunks, who later kills her.
 Plot Involvement
DBZ - Garlic Jr., Trunks, & Android Sagas
Android 18 is woken up, along with her brother 17, by Dr Gero when he runs into trouble from being chased by Vegeta, Piccolo and the others. Shortly after being woken up and told to attack them by Dr Gero, Android 17 instead attacks and kills Dr Gero, incase he ever gets the idea to shut them down again. The two of then decide to activate Android 16 and head off after Goku, who they were designed to kill. While they are travelling around, Vegeta catches up with them and demands a fight, enraged that they ignored him earlier. #17 tells #16 to fight him but he simply refuses, so #18 decides to instead. As Piccolo and the others arrive, #17 warns them not to interfere with Vegeta and #18's fight, or else he will become involved as well.
For the most part they respect #17's warning, until Vegeta's arm is broken by #18 and Trunks steps in to help. #17 reacts and is easily able to knock Trunks, and Piccolo and Tien, down in one hit. As #18 finishes with Vegeta she rejoins #17 and #16 and the three of them head off once again in search of Goku's house. Once they finally reach Goku's house they find it deserted, but #16 muses that Goku is most likely with his friends, either at the Capsule Corp or at Master Roshi's house. As Master Roshi's is nearer, the three of them head there first.
They arrive, but Goku is not there either. However they do find Piccolo amongst the others there who they threaten to kill unless they tell them where Goku is. Piccolo obviously refuses and takes the Androids away from the house so they can fight. #17 steps up to fight Piccolo this time, the two of them fight intensely and seem evenly matched, but before a winner can be decided they are interupted by Cell, who has the intention of absorbing both #17 and #18. Despite the combined efforts of Piccolo, #17 and even #16, they are unable to prevent #17 from being absorbed. Cell begins to change shape and becomes much stronger from absorbing #17, he then turns his attention on absorbing #18 so that he can become perfect. Tien intervenes this time to give the two Androids a chance to escape which they take and hide on one of the many surrounding islands.
They lay low for a while until Cell begins destroying the islands in attempt to flush them out. While they are hiding, Vegeta and Trunks arrive to challenge Cell, and #18 is surprised by how much stronger Vegeta has become since they last fought. Krillin is also nearby, making himself known when he destroys a controller that would deactivate #18. Vegeta and Cell's fight comes to an end with Vegeta becoming bored with Cell's lack of strength and tells him to absorb #18 so that he will become stronger and more of a challenge. Despite the efforts of #16, Krillin and Trunks, Cell is successful and manages to absorb #18.
DBZ - Imperfect & Perfect Cell Sagas
During SSJ2 Gohan's fight with Cell he causes so much damage to him that Cell coughs up Android 18. She is passed out for the entireity of the fight, but once the fight is over she follows the Z Fighters to Kami's Lookout where she overhears Krillin make a wish for the two Androids to be turned back into humans, but Shenlong says this wish is beyond his power. Krillin then decides to wish for them to have their bombs removed instead. She doesn't make herself known until Krillin mentions to his friends that he wants #18 to be happy with #17, at which point #18 points out that #17 is infact her brother, not her boyfriend. She then heads off on her own.
DBZ - Babidi & Majin Buu Sagas
It's been seven years since the defeat of Cell and during that time Android 18 has married Krillin, they also have a young daughter named Marron and have been living at the Kame House. When they hear about the tournament being held, and the prize money involved, #18 decides to enter and forces Krillin too as well.
The day finally arrives and everyone gathers at the tournament, Goku and the others all make it through to the final rounds, as well as Videl and a few other strange characters. The Youth Division is first and as everyone suspected, Goten and Trunks make it to the final. It's quite close, but eventually Trunks is the winner. During the adult matchs, Piccolo decides to forfeit his round, as he senses his opponent is none other than the Lord of Lords himself, the Supreme Kai. Videl gets badly injured during her match and Gohan's is next. He is faced by the Supreme Kai's companion, Kibito, who tells him he wants to see his full power. Gohan agrees and powers up to Super Saiyan, but before they can fight Spopovitch and Yamu, two other fighters who made it to the finals, attack and steal all of Gohan's power and run off. Supreme Kai explains to them all that a wizard has appeared on Earth to ressurrect a monster known as Majin Buu, and that they will need his power to help stop this from happening. Android 18 stays behind to paticipate in the contest, there now only being 5 contestents left. They decide to finish the tournament with a 'battle royale' between the 5 left, Android 18, and Trunks and Goten disguised as the Mighty Mask quickly knock two of the other contenders out of the ring, leaving just themselves and Mr Satan left. Mr Satan stays out of the fight while #18 and Mighty Mask fight fiercly, she soon realises what is going on and exposes the two kids, who fly off after being caught, leaving just her and Mr Satan. She challenges Mr Satan and when she has him in a headlock tells him she will throw the match if he gives her the first place prize money, as well as the 2nd place prize money she would get for coming second. To preserve his image Mr Satan agrees to #18 plan.
After the match, #18 rejoins Bulma and the others and follows them to find the Dragon Balls after Vegeta destroys the tournament ring and later follows them up to the Lookout for safety. She suffers the same fate as they all do up there in that she is eaten by Buu, and later revived by Vegeta's wish. She also contributes her energy to Goku's Spirit Bomb.
10 years after the defeat of Buu and #18 is still living peacefully with Krillin and Marron at the Kame House.
DB GT - Baby Saga
When the world comes under the control of Baby, Android 18 is no exception. She, along with Krillin and Marron, queue up to get on the spaceship to be taken off Earth to the new Planet Plant but just miss the cut off. She's disappointed that Krillin wasn't able to get them on the spaceship. After the defeat of Baby, Android 18 is turned back to normal with the Sacred Water which is released across the Earth.
DB GT - Super 17 Saga
When the hole in Hell is made and several enemies of the past wreak havoc on Earth, Android 18, Krillin and Marron are seen to be trying to escape from the city when they are confronted by Android 17. He asks #18 to rejoin him as his partner and serve Dr Gero again, which she refuses. Android 18 can't understand why #17 would want to join up with Dr Gero again as she remembered that he hated him. #17 then retaliates and kills Krillin and threatens to kill Marron, #18 becomes enraged at the death of her husband and attacks #17, but he is too strong and she is knocked unconscious.
Later, after she regains consciousness and finds Goku fighting with Super 17. She tells #17 that she knows the reason he let her live earlier was because of the bomb inside her. She dares #17 to attack her, saying that the explosion from the bomb would destroy them both, #17 seems hesitant to attack. She then attacks him with ki blasts, keeping him busy while Goku delivers the final blow, destroying him. After the dust settles, Goku explains to #18 why #17 was acting the way he was.