Cell is the ultimate creation of Dr. Gero. Unfortunately, he is pure evil. The Cell of the present DBZ timeline is actually destroyed but the Cell from Trunk's timeline comes back to become complete. After absorbing 17, he became unstoppable, for a while. Vegeta almost beat this villain but the prince was to proud not to let Cell become complete. After completion, Cell defeats Vegeta and Trunks. Then, he starts the Cell games. During the games, Cell is finally defeated by Gohan after becoming SSJ 2.