While a number of the Zone's ghosts were tormenting Porker Lewis in Rouge the Bat's interrogation attempt, King Boom Boo remained inactive. When Knuckles refused to give Rouge the code for the box containing the Chaos Emeralds, she smashed a magic hourglass, which summoned the King of Ghosts. King Boom Boo first appeared before Big the Cat and Buddy (but left them unharmed), gathered the Zone's ghosts to him, and made his way to his summoner. Only Rouge could call him off, and she had blocked the exit tunnel. The only feasible option at that point was for Knuckles to give Rouge the code. With the Emeralds and code in her possession, Rouge and her Badniks made their escape, leaving Knuckles, Porker and Tikal at King Boom Boo's mercy. As the ghost reared up behind them, Knuckles ordered his friends to remain absolutely still. It worked like a charm, as without Rouge to command him, King Boom Boo just couldn't be bothered to attack and led the ghosts elsewhere.
What King Boom Boo is doing now is anyone's guess.