Every hero needs a villain. Not only that, but in order for us to engage in our protagonist's journey, it's crucial that we seem them continue to change and develop in some way or form. In the anime series, Dragon Ball Z we get plenty of this: developments, forms and, of course, villains.
The television series chronicles multiple sagas as the likes of Goku, Vegeta and Gohan fight to defend the Earth from invasion and destruction, with the help of their companions. Firstly encountering the ruthless Frieza, before the being warned of a future threat by the vengeful Dr. Gero and his Androids. Cell would appear soon after, then finally the demonic Majin Buu would emerge.
Each antagonist was more powerful than their predecessor, presenting new challenges to the Saiyan heroes. To overcome them all, they too would have to grow stronger, learn new techniques or even sacrifice themselves for the greater good.
The protagonists never feared. As Vegeta once said "I do not fear this new challenge. Like a true warrior, I will rise to defeat it."
Time for you to meet your challenge.