Android 18 was originally named Lazuli when she was an ordinary human. She is Dr. Gero’s eighteenth version of android creation, designed to serve Gero’s revenge against Goku and she is also the elder twin sister of Android 17. She was human once but Dr.Gero remodeled her and added cybernetics.
Android 18 had certain expectations. While her interests do not initially deviate from this expectation, her curiosity to activate Android 16, in spite of Gero’s orders not to do so, leads Android 17 to take it upon himself to murder Gero. Eventually, Android 18 becomes a member of the Z Fighters, as well as the wife of Krillin and the mother of their daughter Marron.
The androids go to Goku’s home and do not discover him there, leading them to journey to Kame-Sennin’s house, but they do not find him there either due to his having departed not long before.
Android 18 watches Android 17’s battle against Piccolo, at one point becoming impatient and offering her assistance to her brother’s decline, until Cell arrives, who reveals his intent to absorb the pair.
Piccolo is dispatched by Cell. Android 17 is absorbed. Android 18 is nearly being defeated and to be absorbed, as well by Cell’s impersonation of Android 17 until he mistakenly mischaracterizes him. Androids 18 and 16 are given time to escape and they both hide on an island. Android 16 refuses to leave his side and watches Vegeta’s fight against Cell before Krillin approaches her with a remote to turn her off in order to save Android 16 from getting absorbed by Cell as he might get even more powerful and it would become impossible to stop Cell’s destruction in the world.
After Gero is killed by Android 17
Android 18 activates Android 16 following surviving a blast meant to kill her from Trunks and the three begin their quest to find and kill Goku. When they encounter Vegeta, Android 18 decides to fight him and easily beats him in battle, receiving only damage to her clothing and breaking his arm. When Trunks, Tenshinhan, and Piccolo become involved, she and Android 17 easily defeat the trio.
Vegeta easily starts defeating Cell and gives him a chance during the battle so that Cell could absorb Android 18. He thinks that he can defeat Cell easily after reaching his final form by absorbing Android 18. Krillin destroys the remote which was given to destroy Android 18.
Cell After absorbing Android 18
The cell becomes more powerful than everyone has thought. He defeats Vegeta and Trunks easily during the battle. For the next ten days, until the Cell Games tournament, 18 remains trapped and unconscious inside of his body. However, a strong blow from Gohan causes him to regurgitate her.
After Gohan defeats Cell in the Cell Games Tournament
she is healed and declines Krillin’s advances for the time being, despite him using the Dragon Balls to have her self-destruct device removed.
In the alternate-timeline of Future i.e Trunks world, Androids 18 and 17 are mass murderers. Whereas the main timeline versions of the pair never killed innocents. They are identical in appearance to the versions of the two in the present timeline. Android 18 wearing the ensemble she wore in the main timeline upon her activation by Dr. Gero.
From the timeline Future Trunks came, shortly after the death of Goku. They kill their timeline’s versions of Piccolo, Vegeta, Krillin, Yamcha, Tien, and Yajirobe, Gohan managing to escape. Several years later, they kill Gohan and then nearly kill Trunks. Three years later when he confronts the pair during one of their killing sprees. When Trunks returns from the past after fighting with Cell, he becomes stronger than he was in their previous encounter. Android 18 in the alternate timeline shows interest in ending Trunk’s life after confronting her and Android 17. Instead, Trunks killed them both.
In the real timeline, after the Cell Games Tournament, she married Krillin and has a daughter with him named Marron.