Roberto Velasquez was a talented boxer, often sparring with his friend Manuel Torres at Trini's gym, but because of his size, his future as a boxer was uncertain. He was approached by Armando Aviles, who promised to make him stronger and faster, without using drugs. Roberto agreed, and was subjected to the Power Broker treatment in Florida by Dr. Karl Malus. In Roberto's next match, he faced a normal boxer, Emilio Garzon. Without meaning to, Roberto killed him in the ring. Roberto was horrified, and ran out on Aviles, refusing to box anymore.
Manuel was subsequently matched against Rico Lazar, a augmented boxer. This brought Roberto out of hiding to try and save his friend, and Aviles used the opportunity to give Roberto another chance with his crime ring. Roberto refused to do so, and was shot by Aviles. Thinking he was dead, Aviles had his men seal him in a coffin, shoot it full of holes, and toss it into a swamp.
Roberto survived all of this, and swam to safety. Deciding that his "death" had granted him anonymity, he broke into a sporting goods store owned by Aviles, and donned boxing gear, taking the alias of "Bantam." He then went after Aviles, but as he fought him, Aviles was accidentally shot dead by one of his own men. Bantam fled the scene of the murder.
When Roberto went to check on Manuel, he was horrified to learn that he was dead, thrown out of a window by Lazar. As the Bantam, he set after Lazar, and found him at the site where Malus had augmented him. He defeated several of Lazar's allies, but went into a berzerker rage, attacking Captain America. After he had cooled down, he joined Captain America in chasing after Lazar, and Bantam defeated him in battle.
Bantam went back to Trini's gym to work out and was found there by Trini, who had guessed that Roberto was the Bantam. He offered Roberto a career with him as a boxer, but Roberto turned him down, knowing his strength made him too dangerous. Trini then asked him to become a trainer, and Bantam responded that he would think about it.
Later, Bantam came upon two gangs about to fight, and convinced both sides to stand down. He convinced some of the gang members to settle their frustrations at Trini's gym, and ultimately accepted Trini's offer to become a trainer.
Bantam split his time between super heroics and training other boxers at Trini's Gymnasium in San Juan. Because he was too powerful he never stepped himself into the ring again.
After the Super-Hero Registration Act was passed, Bantam chose to register. He came from L.A. to New York to apprehend the hero Thunderclap, who didn't register. Bantam tried to beat Thunderclap with his fists, but Thunderclap used his sonic boom and accidentally sent Bantam flying into a gas truck, which exploded. Bantam was killed by the explosion.