The Dragon will occasionally commit a Heel Face Turn when confronted by the heroes. If he does so, chances are very good that he will end up killing the Big Bad when the heroes cannot bring themselves to do so; often by joining the villain in death as a final act of redemption.
The Dragon is an integral part of the Five-Bad Band dynamic. If there's a Quirky Miniboss Squad, the Dragon is often the unofficial leader.
Please note that effectively being the Big Bad's secretary, or just constantly hanging around them is not enough to be a Dragon. Like The Lancer for The Hero, the Dragon is somebody the Big Bad can rely on (or thinks they can rely on) in a time of trouble to step up and beat the living heck out of the Hero, or maybe just take charge for a while, should the Big Bad not be around to do the job.