Future Android 18 (未来の人造人間18号, Mirai no Jinzōningen Jū Hachi-Gō) (Future Cyborg 18) is the alternate timeline counterpart of Android 18 and is first seen in the special "The History of Trunks". This version of Android 18 is seen as being a more cocky and evil than the present Android 18, bent on doing nothing more but destroying, killing, and shopping sometimes.
In the Anime
In contrast to her main timeline self, Future Android 18 and her brother are extremely sociopathic and cruel, taking joy in killing and destruction. According to them, their personalities are the direct result of Future Dr. Gero's attempts at programming them to hate humans as part of his bid to conquer the world. Despite her brutal and cruel nature, Future 18 has shown instances of self-restraint, such as scolding her brother for murdering a clothing store salesperson despite intending to let him live. She was also very vain when it came to her appearance. When Future Trunks used his sword to swipe a piece of her hair, she would display even greater hostility, likely because in addition to wanting to maintain her looks, her hair does not grow back.
In Dragon Ball Z Abridged
Future Android 18 basically acts the same way as her anime counterpart, being extremely sociopathic and cruel and likes killing innocent people and causing destruction. However, she does posses some personality traits from her main timeline counterpart in the abridged version, such as being touchy against people using sexist slurs, such as being angry at Future Trunks when the latter states that she hits like a girl (which is ironic considering that Trunks is Vegeta's son and he in the present timeline calls her a smug c**t.) Additionally, she does show some caring for others when she stated that Future Gohan was irresponsible for bringing a child (Future Trunks) into a battlefield, and would scold her brother for some of this actions that she seems unnecessary (like blasting her when she wanted to be the one to kill a defeated Future Trunks; although she did get in the way before Future Android 17 shot the blast).
The androids make their appearance six months after the death of Future Goku by murdering all of the Z-Fighters (except of Yamcha) one by one, leaving the world with very little hope. Thirteen years later, the androids still continue to cause havoc across the Earth by destroying small town after small town, working their way up in destroying larger populations.
One day while attacking SuperWorld and enjoying all of the rides, Gohan (the only surviving Z-Fighter from thirteen years ago) and Future Trunks (the son of Bulma and Vegeta who was an infant when the androids killed everyone) arrives, with Gohan challenging the androids to a fight. When it looks like Gohan (as a Super Saiyan) is gaining the upper hand against Android 17, Android 18 is forced to step in and assists her brother in overwhelming the Super Saiyan in a two-on-one battle. This causes Trunks to rush in to help Gohan only to get easily defeated by Android 18. Just as 18 prepares to deliver the finishing blow on Trunks, Gohan manages to attack her and proceeds to hide with Trunks inside the ruins of the demolished theme park. After failing to find the two Saiyan hybrids due to the inability of sensing life energy, the androids decide to bomb the whole area with "Scorched Earth", causing Gohan to lose his left arm as a result.
Later while destroying another city, Gohan ambushes the androids and challenges them to another battle. Amused, the androids taunts Gohan by making a couple of hand-puns before murdering him by shooting down a barrage of ki blasts from he sky. This event eventually causes Trunks to finally become a Super Saiyan due to the grief and anger of losing his mentor and best friend.
Three years later, the androids are attacking Cuminopolis before Trunks, now in his Super Saiyan form, arrives and challenges the androids to a fight. At first, it looks like everything is going well for Trunks until androids quickly turn the tide of the battle and easily dominates him. After beating him down to his normal form, the androids recognizes Trunks as the person they encountered at SuperWorld three years ago with Gohan. Just when 17 proceeds to finish Trunks off with a large blast, 18 suddenly appears in front of him at the last second and gets blasted at point-blank range by he brother. This greatly annoys 18 as she proceeds to leave the city with 17 trying calm her down as Trunks gets left behind, surviving the encounter with the homicidal androids.
After Future Trunks returns to the future after destroying Cell in the present timeline, 18 and 17 are seen at Parsley Town. While 18 was busy destroying buildings because someone called her a "hoebot", 17 went on to kill a civilian that shot at him. Future Trunks then arrives at the scene in his Super Saiyan form, with the intent of finally destroying both 18 and 17. While 18 tries to fight Trunks, she ends up getting beaten easily and is later destroyed by him, with her brother following shortly afterwards.
Android 18: I can't believe he brought a kid to a battle! Super irresponsible.
Trunks: (narration) She wasn't wrong.
Trunks: I'm not afraid of you! I can take care of you myself!
Trunks: (narration) I, however, was.
(Android 18 punches Trunks then grabs him by the shirt)
Android 18: Now what do you think?
Trunks: You hit like a girl!
Android 18: *sighs* Well, you're not wrong. -- "The History of Trunks"
Android 17: Well, shit... Think he got away?
Android 18: I don't know... We can't sense life energy...
Android 17: Seriously, what was the point of not giving us that ability? Seems like an intentional design flaw, if you ask me.
Android 18: Oh, well. Scorched Earth?
Android 17: Scorched Earth.
Gohan: (thinking) Aw, crapbaskets! -- "The History of Trunks"
(Trunks has just got defeated by the androids and has reverted to his normal form)
Android 17: Wait a sec... This kid look familiar to you, sis?
Android 18: Yeah! He was at the amusement park with Stumpy!
Android 17: (begins smashing his foot into Trunk's face) Aw, who's a little survivor? You are, yes, you are! (raises his hand) Let's fix that, shall we? (prepares a blast)
Android 18: (suddenly zooms in front of the blast) DIBS!
Android 17: SIS, GET OUT OF THE-- (the blast detonates)
Android 18: Did you just friggin' blast me?!
Android 17: You stepped in front of it, what is wrong with you?!
Android 18: No, no, that's it! We're leaving!
Android 17: Oh, come on, sis... Sis, get back here!
Trunks: (narration) I still have no idea how I survived. -- "The History of Trunks"