Dr. Gero

Dr. Maki Gero, also known as Android 20, is the main antagonist of the Android Saga in Dragon Ball Z and one of the two main antagonists of the Super 17 Saga in Dragon Ball GT alongside Dr. Myuu. He is an evil genius scientist and a brilliant weapons designer behind the criminal organization known as the Red Ribbon Army. As an android, he can drain energy from people through a device on his hand. Being the one responsible for creating the androids and Cell, as well as Super 17.He is also the creator of Android 21 who was originally the mother of his son.

He was voiced by the late Kōji Yada in the Japanese version, and Kent Williams in the English version.

Personality
Dr. Gero is a cold-hearted and merciless man who is willing to stop at nothing to achieve his goals, he doesn't let people get in his way, and even kidnaps other people to use for his experiments. He also holds a strong grudge against Goku due to him destroyed the entire Red Ribbon Army. He has the utmost confidence in his technological skills and intellect, as he believes, even when the odds are against him, that he, or his creations, will succeed somehow. Gero also trusts his creations with his life, believing now that he has "improved" them, they will respect him as their master. However despite his cold heart, he is capable of some affection, as he modeled Android 16 after his deceased son and deliberately made 16 gentle in order to avoid the risk of him being destroyed in battle. As he is unable to bear the thought of that happening.

However, he had trouble with his creations, Android 17 and 18, for disobeying him due to them being arrogant and rude. Arguably, he can also be quite hypocritical and self-righteous, because even if he despised Son Goku for destroying the Red Ribbon Army for 20 years he used Goku's DNA to create Cell, whether this occurred before or after Son Goku destroyed the Red Ribbon is debatable.

Dragon Ball
Maki Gero was the chief scientist of the Red Ribbon Army (though he is never seen or even mentioned at the time) that created the first androids in the service of the Red Ribbon Army. In the anime, Dr. Flappe (who looks nothing like Dr. Gero) is named as Android 8's creator. A possible explanation is that Flappe and Gero were partners and constructed the first androids together (most likely including Android 8) or that Flappe merely constructed Android 8 while Gero produced the blueprints and forced him to create him (most likely the latter as Flappe was hinted at being a Red Ribbon member against his own will).Akira confirmed them to be partners and he modeled Android 16 after his late son who was a high-ranking member who died in a gunfire.

After the Red Ribbon Army was defeated by Son 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 Warriors 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 to 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 Artificial Humans (Androids).