Bahamut is the god-king of the Dragons and had spent years slumbering until the Light Warriors woke him up. Like his original counterpart, he agrees to give them a class change if they bring him a Rat tail. It is later revealed that this is in fact an ingredient for a virility potion to be made by Bahamut's girlfriend Matoya, much to the disgust of the Light Warriors. Bahamut upheld his end of the bargain and increased the Light Warriors powers, which in turn led to their eventual class change shortly afterwards.
A recurring gag throughout the series, Thief's adamant refusal to believe in dragons despite meeting so many, began with meeting Bahamut. According to Muffin, an ancient prophecy said that Bahamut would sleep until the end of the world and would awaken upon its imminent doom, alerting Red Mage to the true peril of the world that they may have accidentally caused by awakening Bahamut. However, the other Light Warriors pay him no mind. What part, if any, Bahamut will play in the supposed end of the world is unclear.