The ultimate creation of Dr. Gero. After extensive research of
gathering information about the Namic, Saiyajin and Freeza's race's fighting
techniques as well as their biological make-up, Dr. Gero was able to combine the
best of everyone into a single entity called Cell. Cell's purpose seemed to be
nothing but to do be ultimate fighting machine regardless of the methods (such
taking millions of lives to fuel his power).
Cell's power is not fully realized until he absorbs both #17 and #18, which is
called the "Complete Form" or the 3rd stage of development. After he
had reached Complete Form and believed that no could defeat him, everything
became a game to him.
He subsequently spared the life of Trunks after Trunks forfeited the fight
against him. He announced the Cell Game and gave the Dragon Ball Warriors time
to "get up to speed" in order to seek more challenge.
He had always believed that Son Goku was his ultimate challenge. It wasn't until
Son Goku forfeited the fight and handed it over to his more powerful son, Gohan
(no pun intended).
When Cell was finally confronted with defeat and even death by Gohan, nothing
was a game to him anymore. Vegence was the only thing that filled his evil mind.
He became much more powerful after his unsuccessful attempt to destroy the world
by his self-destruct mechanism. But he was still no match for Son Gohan