Dr. Gero

""I cleared the area of innocents, in accordance with your wishes. Do you disagree with my methods?""

- - Dr. Gero

Dr. Gero, also known as Android 20, is an evil genius and a brilliant weapons designer behind the criminal organization known as the Red Ribbon Army, and he is one of Goku's arch enemies in Dragonball Z and Dragonball Z Kai. As an android, he can drain energy from people through a device on his hand.

Dragonball
Dr. Gero is the head scientist of the Red Ribbon Army (though he is never seen at the time) that created the first androids in the service of the Red Ribbon Army. In the anime, a Dr. Flappe (who looks nothing like Dr. Gero) is named as Android 8's creator. A possible explanation is that Dr. Flappe and Dr. Gero were partners and constructed the first androids together (most likely including Android 8) or that Dr. Flappe merely constructed Android 8 while Dr. Gero produced the blueprints and forced Flappe to create him (most likely the latter as Flappe was hinted at being a Red Ribbon member against his own will). After the Red Ribbon Army was defeated by Goku, Gero went into hiding in a mountain area that was north of North City where his laboratory was located. In this lab, he strived to create the ultimate fighting machine. Using an extremely intelligent tracking device disguised as a ladybug, he studied Goku and the Z Fighters for years on end: the fights Goku had with Tenshinhan, Piccolo Daimao, Piccolo, and the Saiyans Raditz, Nappa and Vegeta.

Dr. Gero ceased the study of Goku's power when the Saiyan left for Planet Namek, believing he had already compiled enough information; so he simply adjusted the androids' power levels to accommodate for Goku's expected power increase, not accounting for the power level of Goku's Super Saiyan form. Believing he had created the ultimate fighting machines, he finalized his plans and constructed a series of Androids.

Dragonball Z
Dr. Gero sought revenge on Goku, so he studied Goku and all his fights on Earth, including his fight with Nappa and Vegeta. He built androids with as much power as an army so they could defeat Goku. It is said by Trunks that the androids killed Dr. Gero when they awakened (as this was what happened in his timeline). However, this did not immeditely happen, as Dr. Gero awakened Android 19, who made him into Android 20. They attacked a city where Goku and the other Z Fighters (minus Vegeta and Chiaotzu) were waiting for them. Android 20 destroyed a city to prevent the people from getting in the way of their fight. In a deserted area, 20 sent 19 after Goku, who then became crippled with a heart virus, and was forced to leave with Yamcha.

After Vegeta arrived and destroyed 19, 20 ran, deciding to steal energy from the other Z Fighters so he would be strong enough to defeat Vegeta. He targeted Piccolo first, but Gohan stopped him, and Piccolo beat up 20, cutting off his hand. However, Trunks showed up and revealed that 19 and 20 were not the androids from his timeline, but 17 and 18 were. 20 retreated to his lab, but attacked a man along the way who shot at him mistaking him for a deer, leading Kuririn to fight him. They followed him to his lab where he awakened 17 and 18, who disliked being controlled and took his remote and crushed it.

20 tried to stop 17 and 18 awakening Android 16, saying it was too dangerous to awaken him, but 17 impaled him with his hand and kicked his head off. He survived, so 17 stomped on his head, crushing it.

Dr. Gero's master computer was creating an ultimate android Cell from the cells of Goku, Gohan, Piccolo, Yamcha, Tien, Chiaotzu, Vegeta, Nappa, Frieza, and King Cold, which Gero had extracted using an insectlike device. In Cell's timeline (and possibly Trunks's timeline as well), Gero was killed when 17 cut his head off and 18 destroyed his head after he said Cell would swallow them up. In the regular timeline, the master computer and Cell in his larval form were destroyed by Kuririn and Trunks.

He later appeared in Hell alongside several other dead villains during the Kid Buu Saga, watching Goku's fight with Kid Buu.

He also appeared in the movie Super Android 13, during his death at the hands of Android 17 (though the dialogue is changed), with his master computer programmed to think it was Dr. Gero, and it finished building Androids 13, 14, and 15. In the anime (as most of the movies are non-canon) it is said by 18 that all androids up to 15 were destroyed by Gero (excluding Android 8, as Gero never built him).

Dragonball GT
Dr. Gero and Dr. Myuu worked together to build another Android 17 in Hell, brainwashing the original 17 (who was no longer evil) to open up a portal from Hell which most of the dead villains escaped. The two 17s fused together into Super 17. Gero was betrayed and killed by Super 17 with an energy wave.