In the fictional manga/animes Dragon Ball and Dragon Ball Z, there are several Androids.
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Eighter[]
In Dragon Ball, the first Android we see is Android 8, a robot that resembles Frankenstein. Agent Purple (Murasaki) uses him to fight Goku, but Android 8 refuses. When Purple threatens to press his self destruct button, Goku knocks it out of his hand. Goku nicknames him Hachan or Eighter, and he helps defeat General White. Eighter gives some energy to Goku to power his Spirit Bomb.
In the anime, Dr Frappe was credited with building Android 8. However, this is incorrect, as it was actually Dr Gero.
Dr Gero[]
Dr Gero was the supergenius technical leader of the Red Ribbon Army. However, when Goku destroyed the whole Army, Gero managed to survive. He then dedicated his life to revenge, and built more Androids in order to destroy Goku and his friends.
Gero created a device similar to a ladybug to follow Goku around and track his life, as well as his friends. Using the information gathered, Gero built two super powerful teenage twin Androids, called Android 17 and 18.
The Alternate Future[]
When he released these Androids, however, they turned on their master and destroyed him. They then decided they may as well carry out his plan, to destroy Goku and his friends. However, Goku was already dead (he had died a year or so ago due to a heart disease). So the evil pair killed all of his friends except Son Gohan, Trunks and Bulma. And with Piccolo dead the Dragon Balls were gone so nobody could be wished back.
The evil pair then proceeded to virtually destroy the world and its inhabitants. Gohan tried to fight them, but they were far stronger than he, even as a Super Saiyan. Gohan tired to teach Trunks to become a Super Saiyan too, but he couldn't. Gohan was eventually killed too by the Androids, which made Trunks finally become a Super Saiyan.
Bulma had built a time machine so Trunks could go back and give Goku an antidote to his heart disease. Trunks travelled back in time, and warned Goku and his friends bout the Android menace.
The Androids Saga[]
The Z Senshi trained for the next 3 years in order to defeat the Androids. Vegeta managed to become a Super Saiyan. They found the Androids and fought them, but unbeknownst to them these were not the Androids Trunks had warned them about.
New Androids: 19 and 20[]
Instead, they faced Android 19, a fat, white, purely mechanical android, and Android 20, an old-looking android built into a human. These two androids were not as powerful as the ones from Trunks' time.
Goku's heart disease had then kicked in, so he was nearly killed by Android 19, but Vegeta saved him and destroyed 19 easily. Yamcha returned Goku home. But before Piccolo could destroy Android 20, he escaped. 20 was, in fact, Dr Gero himself! Gero had built 19, then instructed 19 to make him into an Android to increase his strength. Gero now planned to release his more powerful Androids, 17 and 18.
Trunks had returned from the future, and told the Z Senshi they had the wrong androids, and described 17 and 18 to them.
They're stronger, and there's more of them[]
But when Gero released them, they destroyed him just as their future counterparts had. But this time there was a third android, 16. However, 16 refused to fight anyone but Goku. Trunks also noticed this 17 and 18 were much stronger than the ones he knew.
17 and 18 beat up Vegeta, Piccolo, Trunks Tenshinhan but left Krillin alone. They then took off in search of Goku.
A New Threat: The Cell Saga[]
Dr Gero's ultimate creation now revealed itself. Its name was Cell, a biological android created from the DNA of all the Z Senshi, as well as some of the more notable villains.
Cell is from a third timeline, where Trunks was able to destroy 17, and 18. In order to reach his full potential, he needs to absorb 17, and 18. As Trunks prepared to return to the past, to inform the Z fighters of his accomplishment, he was ambushed by Cell. Cell then reset the time machine to go back 3 years earlier than Trunks was going. He then reverted to his larval state, so that he could fit into the time machine. The extra 3 years ensured that he would be able to grow back into his humanoid form, and possibly be able to stop Trunks from destroying 17, and 18 in the past as well. It was this act which created the timeline where 16, 19, and 20 exist, the timeline as we know it.