Neppa is a man with a large build and overly-muscular arms, which are wrapped with bandages at the wrist. Compared to the rest of his body, he has disproportionately smaller legs. His eyes are narrow with a triangle-shaped nose, and his eyebrows are constantly furrowed. His hair is stylized in a spiked pompadour which spreads out onto his face in wide, arrow-shaped sideburns.
His attire consists of a light-colored shirt with frills at the neck, which he wears under a dark vest. This is all covered with a sleeveless jacket that is designed with fur around the neck, with the words "ACID RULE" written on the back. Below this, he wears a regular pair of dark-colored trousers that are held up with a light-centered belt, and he wears a pair of dark shoes, with each shoe featuring a buckle near the back.
He has a peculiar habit of having a soda bottle clamped between his teeth at all times, and does not relinquish hold of it even when engaged in battle. He seems to take great pleasure in utilizing his Magic, often urging his opponents to melt underneath its acidic power, and expressed great surprise when Pantherlily actually managed to cut through it. Despite being an executioner, he displays concern for his comrades' safety as he tried saving Uosuke from the shadow that is swallowing him by holding his hand and telling him not to go, despite of sinking in as well. When he speaks, he usually ends his sentences with "Pan".
Acid Magic: A Caster Magic that allows Neppa to create acid; it was specifically designed in order to kill people. This substance seems to be created from the Knight's fist, as seen when he punched the ground and sent a stream of acid spiraling towards Natsu and company, melting the area. The acid has been seen to appear from the ground in a straight line towards the user's opponent, from Neppa's fist in a spiraling motion, and also as a horizontal tornado directed towards the target. The acid produced is extremely corrosive, to the point where it can gnaw through solid stone, and has been noted to be extremely difficult to deflect or cut, making it an ideal weapon against most opponents.
- Multi-Angle Acid: Neppa creates numerous purple spheres made of corrosive acid, wherein he is capable of ordering them to rush towards his targets.
- Acid Pillar: Neppa slams his fist on the ground, causing it to erupt a pillar of acid or several more below the target.