Zidane
Up until when Zidane was born, Kuja was Garland's most powerful genome. However, Garland realized that Kuja was much too powerful, so he created a new genome -- Zidane. When he was brought to life, Kuja could not bear to see a genome with more power than himself -- he naturally felt threatened. Kuja decided to take matters into his own by dropping Zidane on the planet Gaia -- the planet he had been ordered to destroy. Zidane would have also been in charge of destroying Gaia, but if Kuja could do it on his own, he felt that he would prove his superiority.
Naturally, when Kuja stumbles upon Zidane in Burmecia, he realizes that Zidane could be a bit of a problem. However, Zidane's existence seems to make Kuja's quest to ruin Gaia even more fun for him. Now he can torture the one he hates and his loved ones through what he does. By fighting Zidane and manipulating him, Kuja is able to boost his ego. Garland always said Zidane was better, but doing these things, in Kuja's mind, proves that Garland was wrong.
Even though by this point Zidane doesn't know anything about how he and Kuja are connected, he nevertheless hates him. All he knows is that Kuja is messing with Queen Brahne's head and is out to do no good. Since Zidane is a natural do-gooder, he takes it upon himself to save the world from Kuja.
Once Zidane finally follows Kuja to Terra, their home planet, Zidane learns the truth of his existence. He learns why he was created and that he and Kuja are essentially brothers. He rejects this information, saying that he and Kuja are nothing alike, that he is nothing like the other genomes, but Garland comments that Kuja said the exact same thing. He also tells Zidane that Kuja's life is limited -- Zidane was created to eventually take his place.
When Kuja learns about his fate, he decides that everyone must die with him. Unfortunately for Kuja, this is not the case. Zidane and co. defeat him, and the Iifa Tree kind of swallows him up. When Zidane is about to flee, he hears Kuja's voice -- not because he is near, but because they are connected somehow. He decides to turn back, despite the complaints from his travel companions.
When Zidane finds Kuja, Kuja tells Zidane that he realizes he was wrong, that he doesn't deserve to live after all he did; he realized the meaning of living, but he was too late. And there Kuja dies, lying in Zidane's arms, after the two brothers had finally accepted each other.