Androids 17 and 18
Androids 17 and 18 have been shrouded in some degree of mystery. This brother and sister duo have reeked all kinds of havoc before falling prey to the android known as Cell. They would both move on after the event with 17 seemingly falling off the Earth and 18 marring Krillan and mothering a child with him.
Androids 17 and 18 were once a pair of human twins. 17 was named Lapis and 18's name was Lazuli. They were captured by the then Leader of the Red Ribbon Army Dr. Gero and experimented on for an unknown amount of time. In that process they were converted into infinite energy androids (though cyborgs might be more accurate) as opposed to the energy absorbing models. Unfortunately for Dr. Gero they proved to be troublesome and difficult to control and were put into a deep sleep possibly never to reawaken.
Again an unknown amount of time passes and Dr. Gero (Now and android himself) and another android face off against the Z warriors. Vegeta makes short work of the first android and Dr. Gero gets damaged in the process. Dr. Gero makes it to his lab and in a desperate gamble reactivates Lapis and Lazuli. It doesn't end well for Dr. Gero as Lapis takes his head. Lapis and Lazuli take posession of an another android (Android 16) and proceed to destroy the lab.
They reactivate 16 and end up setting their objective based on Android 16's programming; destroy Goku! Unfortunately for them at this time Goku was suffering from the heart virus that took his life in the future Trunks timeline. Instead they get to spar with the remaining Z warriors making short work of them. Krillan however gets a special surprise from Lazuli.
After some training the Z warriors are able to push back against the androids but a new and greater threat has emerged. The being known as Cell begins hunting the android twins in a bid to increase it's powers. Cell proves even in his basic form to be an even greater threat than 17 and 18 and eventually absorbs them both. The twins are taken out of the equation mostly from that point on. After Cells defeat they return but to more of a common life rather than foes.
Android 17 falls off the face of the Earth basically and you don't hear anything about him until Super where you learn he became a park ranger. As a former human he was also able to increase his power by training even forcing Goku to go super Saiyan blue in a sparring match. Android 18 decides to stick around and give Krillan a break. They end up getting married and have a child named Marron (Not to be confused with Maron; Krillan's bubble headed ex-girlfriend). Like 17, 18 continues to increase her power in Dragon Ball Super.
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