Diavolo is a veritably tall man in his 30's with a lean and semi-muscular build. In his first appearance, covered in shadow in all but his glaring eyes, he appears to have short, combed back straight hair and is wearing a suit and tie.
His appearance is drastically altered when fully revealed: Diavolo has long hair, leopard-patterned with various spots, parted in a zig-zagging manner; the bangs of which are propped and parted slightly to the side. The rest of his hair is cut straight across at his shoulders. In earlier depictions, varying amounts of Diavolo's hair were cut straight, inconsistently ranging from small locks to the entirety of his hair. His shirt, hidden under Doppio's sweater, appears to be made of complex mesh-wiring: it covers his shoulders and torso but down his back and abdomen are linked circles that are tucked under his belt, which in the back end in two~three long "coat tails". On his arms are elegantly designed floral sleeves leading down to a pair of wristbands. He also shares a pair of studded pants and seemingly sock-less loafers with Doppio. In most of his appearances, Diavolo has a slightly hunched posture when standing.
His eyes are unique as his irises are "fragmented", forming as spots and shards of whole irides with a triangular shaped highlight to them. However, while Bucciarati is in control of his body, they become normal with circular highlights.
His appearance differs greatly while operating as his alter-ego, Doppio. As Doppio is a completely separate soul sharing a body with Diavolo, they even have different muscular builds.
The series is known for alternating colors between media, the information presented below may or may not be canon.