Dr. Gero

From Multiversal Omnipedia

Dr. Gero was originally one of the chief scientists of the Red Ribbon Army. He swore revenge on the young hero Goku after he singlehandedly destroyed Red Ribbon, and spent the following decades creating the perfect weapons to destroy him. Gero's initial plans involved the creation of androids. However, his plans adjusted after Goku's battle against Piccolo at the World Martial Arts Tournament. Gero decided that he would need the ultimate warrior to defeat Goku, and took cell samples over the years of Goku, Piccolo, Vegeta, Nappa, Gohan, Frieza and King Cold (and probably Tien, Krillin and Yamcha as well). Gero planned to combine and improve upon the cells for his creation.

In the meantime, he continued to focus on machines, culminating in the powerful Android 16. After 16, he pursued another avenue, creating a pair of androids from a pair of human twins- Android 17 and Android 18. However, the twins were dangerously rebellious, so Gero placed them in stasis. Instead, he planned for them to become the means by which his ultimate warrior- later known as Cell- would ascend to his perfect state, by absorbing them.

Gero's next experiments were on himself. Creating Android 19, he used it transfer his brain to an android body, Android 20. Equipping himself and 19 with energy-absorption devices, he felt he was ready to destroy Goku. However, Goku and friends had been warned about Gero's plans by Trunks. When Gero unleashed his attack near South City, the Z Warriors were ready to face them. Despite that, 19 and Gero were able to nearly kill Yamcha and Goku, until Vegeta- now a Super Saiyan- came onto the scene. 19 was destroyed, and Gero retreated to his lab, deciding that he could force 17 and 18 to defend him. Unfortunately for him, they decided instead to kill him instead- 17 beheaded him and crushed his brain.

Dragon Ball GT

Winding up in Hell (HFIL), Gero eventually joined forces with Dr. Myuu to create Artificial 17, who used the real 17 to open a portal to Earth. Artificial 17 then combined with the real 17 to form Super 17. Myuu, however, no longer had any need for his partner, and had Super 17 destroy Gero once again.

Alternate Timelines

In the future that Trunks came from, Gero survived longer, only being slain by 17 and 18 after he revealed Cell to them.

Comments

It's unclear whether Gero intended to make 17 and 18 part of Cell all along, or if he incorporated 17 and 18 into his plans for Cell after they rebelled.