Vinegar Doppio is Diavolo's young, innocent, and bizarre second personality. He considers himself as Diavolo's most trusted subordinate. Doppio acts as a disguise for Diavolo, when he interacts socially with others in the outside world. He turns into Diavolo whenever someone finds out about Diavolo's identity. Doppio communicates with Diavolo with an object that only Doppio hears ringing, making people think that he's insane, or scary (to what children think of him). Doppio usually makes the sound とぅるるるる~ (Turururu~), which is similar to a phone ringing.
Doppio means "Double" in Italian, showing his nature as Diavolo's second personality.