Android 20

Dr. Gero, also known as Android 20, is an evil scientist, and one of Goku's arch enemies.

Dragonball
In Dragonball, Dr. Gero was the leader and founder of the Red Ribbon Army. He built several androids, but Android 8 was created by someone else named Dr. Flappe. Ultimately, the Red Ribbon Army was destroyed by Goku.

Dragonball Z
Dr. Gero seeked 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 an android, Android 20. They attacked a city where Goku and the other Z Fighters (minus Vegeta and Chiaotzu) were waiting for them. Dr. Gero destroyed a city to prevent the people from getting in the way of their fight. In a deserted area, Dr. Gero 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, Dr. Gero 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 Dr. Gero, 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. Dr. Gero retreated to his lab, but attacked a man along the way who shot at him mistaking him for a deer, leading Krillin 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.

Dr. Gero 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 Dr. Gero had extracted using an insectlike device. In Cell's timeline (and possibly Trunks's timeline as well), Dr. 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 Krillin 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 14, 15, and 13. 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 Dr. Gero (excluding Android 8, as Dr. 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. Dr. Gero was betrayed and killed by Super 17 with an energy wave.