Diavolo/Abilities and Powers

Personal
Transformation: Despite (likely) being human, Diavolo and Doppio share many minor abilities in changing their physical appearance. Doppio looks to be around 17 years of age and shorter than Bucciarati. However, while attacking that same man as Diavolo, he appeared to be a head taller, if not more, as well as being muscular as former protagonists like Joseph - however, during the battle in Rome, Diavolo was roughly around Mista's height. Furthermore, when Diavolo takes over, Doppio's freckles disappear and his voice deepens, and it may be possible that his hair can become longer or shorter than Doppio's. According to Diavolo, it takes him roughly 10 seconds to completely take over Doppio's body, but the transformation back to Doppio is instant.

Various Other Benefits for Diavolo: While Doppio is in control, Diavolo has quite a few abilities, border-lining omniscience. He was able to tell that the taxi driver didn't see the picture of Donatella that Doppio was carrying, despite his subordinate's beliefs, and was later able to see the lurking Risotto. He is also attuned to souls while not in control, such as being able to tell the near dying Bucciarati's state and being able to disguise Doppio as Trish. Diavolo also seems to be able to manipulate Doppio's memories, as Diavolo remembers everything and can see everything that happens to Doppio but not vice versa, although it's unknown how much of this is based off Doppio's possible poor memory.

The "Phones": Diavolo can communicate with Doppio through any kind of object while not in control, granting his subordinate the belief that his alter ego is another person all together. Although we haven't heard the quality of these "calls" yet and whether or not the objects alter the quality of Diavolo's voice, Doppio can always hear his alter ego's voice through the object. Doppio also, on occasion, forgets that he's been able to communicate with Diavolo, but he consistently has never realized he's been using a broken or non-phone object. These "phones" have ranged from ice cream to frogs, to cigarettes to toys, and of course Doppio always carries on him a broken or dismantled phone headset.

Stand
Diavolo's Stand is King Crimson, an extremely powerful Stand with tremendous physical strength and the ability to erase time up to a 10 seconds time frame as well as predict anything that happens over the following 10 seconds. Although it has some limitations, King Crimson renders him essentially invincible in a fight as he can either foresee and counter the enemy's actions or render them meaningless by erasing the timeframe in which they happen.