Vinegar Doppio, a significant side character and secondary antagonist, serves as the underboss to Diavolo. He holds Diavolo in high regard, often addressing him as 'Boss'. However, the true nature of their relationship remains unknown to Doppio. Diavolo utilizes Doppio for private missions and public appearances, maintaining his own anonymity. Diavolo also displays a certain level of affection for Doppio, providing him with guidance and cautioning him against potential threats.