Personality
Matthew is unfortunately, quite self-centered. Though he can on occasion have bursts of altruism, he usually has to be pushed to do so. He's also a thrill-seeker, as anyone who would wear a costume and fight crime -- or commit it -- would have to be. Despite his predilection for thrill seeking, most of the time he's calm and methodical, planning his actions out first before jumping into the fray. Of course, all that carefulness will go straight out the window if he's emotionally compromised, which has been known to happen.
Background
Matthew Stuart lived a normal life until his 13th birthday, when his father, an archeologist, gave Matthew a small relic that was discovered on his latest archeological dig. Matthew, who was angry at his father for missing his birthday party, stormed off to his room, relic in tow. Once there, he discovered that the relic was more than it appeared to be, and broke it in half, releasing an ancient, extremely powerful, evil entity that possessed the boy and used him as a conduit for its power. Under the entity's control, Matthew, now calling himself Bedlam, used his magic to send all the adults on the planet to a parallel world, where they remained, until Young Justice tracked Bedlam down and confronted him. He tried to use his power to give life to their thoughts and make them destroy each other, and it almost worked, until Impulse simply imagined a video game system, which he kept resetting so that Bedlam couldn't do anything. Eventually, this severely weakened the entity, releasing Matthew from its hold, but lapsed into a coma shortly afterward, and was taken into the custody of the DEO (Department of Extranormal Operations). The entity would later apparently die, at least to all appearances, but Matthew retained its powers. He later tried to get revenge on Young Justice, whom he blamed for leaving him in a coma. The attempt at revenge ended with Matthew back in a coma and his powers gone. But he's back now, and his powers are somehow restored.