Android 18 enters Dragon Ball Z as Dr. Gero's weapon and leaves it as Krillin's wife and Marron's mother, without ever losing her cool, practical streak. Android 18's profile covers her origins and powers; here we follow what she does arc by arc, from waking up in the lab to the Tournament of Power.
Dragon Ball Z
Androids Saga (episodes 118-139)
In the Androids Saga Dr. Gero wakes her and her twin, Android 17, in episode 133, with orders to kill Goku. The pair rebel at once: 17 kills their creator and 18 activates Android 16. Then Vegeta shows up, eager to take her on as a Super Saiyan: in episode 135 Android 18, running on unlimited energy, holds her ground and breaks his arm with a kick. In episode 136 she finishes him off while her brother flattens Trunks, Piccolo and Tien. They spare everyone and, in a stolen car, set off to hunt Goku almost as a game, but not before she kisses Krillin on the cheek.
Cell Saga (episodes 140-194)
In the Cell Saga she learns she was also designed as a spare part: Cell wants to absorb her and 17 to become perfect. Cell even tries to trick her by imitating 17, but gives himself away; once her brother is gone she escapes with Android 16 thanks to Tien and goes into hiding. Krillin finds her with the remote that can shut her down and, in episode 158, smashes it in front of her. Soon after, Cell absorbs her and reaches his perfect form. She stays trapped until the Cell Games, when in episode 187 a blow from Gohan forces the monster to spit her out. Krillin uses a wish to remove her bomb, and she leaves with a simple "see you later". In Trunks's future, by contrast, Androids 18 and 17 are ruthless killers, and Trunks wipes them out in episode 194.
Majin Buu Saga (episodes 200-291)
Seven years later she is married to Krillin and has a daughter, Marron. In the Majin Buu Saga she pushes her husband to enter the 25th tournament for the prize money and, at the punching machine, hits softly so as not to draw attention. In the closing battle royal she faces Mighty Mask and, in episode 225, unmasks Goten and Trunks, who are disqualified. In episode 226 she cuts a deal with Mr. Satan: she loses on purpose for 20 million zeni. Sheltering at Kami's Lookout with Marron, in episode 260 she is turned into chocolate and eaten by Buu; once revived, she gives her energy to the Spirit Bomb.
Dragon Ball GT
Baby and Super 17 Sagas (episodes 17-47)
In the Baby Saga she falls under the parasite's control like nearly everyone on Earth and is freed with the Sacred Water. In the Super 17 Saga her brother, manipulated by Dr. Gero, hypnotizes her in episode 43. Krillin brings her back to her senses and Android 17 kills him: in episode 44 she attacks to avenge him and is defeated. She returns in episode 47: she taunts Super 17 and, while he absorbs her blasts, Goku finds the opening to destroy him. Afterwards she is assured that the Dragon Balls will bring Krillin back.
Dragon Ball Super
Battle of Gods Saga (episodes 1-18)
At Bulma's party, in the Battle of Gods Saga, she tries to take on Beerus with Piccolo and Tien, and in episode 7 she is beaten like the others.
Universe Survival Saga (episodes 77-131)
In the Universe Survival Saga she joins Universe 7's team for a ten-million-zeni prize that Goku makes up in episode 84, and reveals where to find Android 17. In the tournament she teams up with Krillin against Shosa, then fights on her own. In episode 117 she seems finished against the giant Ribrianne, but thinking of Krillin and Marron gives her the strength to break free and knock her out. In episode 121, as 17 is about to fall off the stage, she leaps down in his place and kicks him back up, sacrificing herself for her brother. In the manga, after the tournament, she fights alongside her twin against the henchmen Moro sends to Earth.
Movies
In the film Battle of Gods she attacks Beerus and is dropped with a single chop to the neck. In Super Hero she reaches the new Red Ribbon Army base with Krillin and fights Cell Max.
Did you know? Her human name, Lazuli, and her twin's, Lapis, come from the same stone: put them together and you get lapis lazuli.
Our take
Followed arc by arc, Android 18 is one of the few Dragon Ball characters who truly switch sides without changing personality. In the Androids Saga she is the most concrete threat, the one who breaks Vegeta's arm; in the Majin Buu Saga she is a wife who gets paid to lose a tournament. The thread tying them together is her pragmatism, and it works.
GT uses her mostly for the grief of Krillin's death, while Super gives her room to fight again: the win over Ribrianne and the sacrifice for 17 are two of the most human moments of the Tournament of Power.
To complete the picture, revisit the profiles of Krillin, her husband, Android 17, her twin, and Cell, the monster who wanted to absorb her. To follow their stories arc by arc too: Krillin saga by saga and Android 17 saga by saga.