Bojack is the main antagonist. He is the leader of the Galaxy Soldiers. His name is derived from the Japanese word boujakubujin, which means "arrogance" or "audacity.
As stated above, his goal is to conquer the universe and destroy all those that stand in his way. He was sealed by all four Kais in a star. However, according to North Kai, the seal broke when Gokuu teleported the devoluted, ready-to-self-explode Cell to North Kai's planet, killing him and Gokuu at the same time, and allowing Bojack to be freed.
Bojack is shown to have a bloodthirsty personality, as he mentions upon revealing himself to Gohan that it has been a long time since he "snapped someone's neck." He is also known to have little to no loyalty to his own comrades.
Bojack and his crew were designed by Akira Toriyama, the original manga author, for Toei's theatrical production. Toriyama's designs can be seen in Daizenshuu 6.
(Source: Dragon Ball Wiki)
