Mard is tall, lean, yet fairy muscular, and has the appearance of a young man whose age is mid-twenties with a slightly pale skin tone. His black hair is long and wavy, reaching his shoulders even whilst the majority is tied up in a very large ponytail. Additionally, his messy bangs all but obscure his dark crimson, slanted eyes.
For clothing, Mard's dons a long, black jacket trimmed with a dark-blue flame pattern; the lapels are a matching lighter color as well. The cuffs of his jacket are rolled up, revealing a light-colored inside, which contrasts the black outside of his jacket, and have a black line cutting through the edge of the fabric. The jacket itself is tattered and splits into four torn fabric segments at the tail. Beneath the jacket, Mard wears a frilly black V-necked shirt, lined in a lighter color, that closes down the middle and is adorned with a buckle just below his clavicle; just below this is a piece of fabric that Mard keeps over this shirt but under his jacket, that circles the majority of the lower half of his torso. The rest of his attire is simple, consisting of light-grey pants that lay lazily over the top of rather plain black boots and have, on each leg, a seam running down the middle, as well as two pairs of buckles that form an X on the thigh and lower calf. However, the buckles on each thigh later disappear, leaving only the ones on each calf.
Mard is a cool, aloof, soft-spoken, collected, yet arrogant and ruthless man, who plays himself off as being extremely intelligent; he quite regularly refers to himself in the third person. He also possesses a macabre sense of humor when mocking his opponents in battle.
Also, much like the rest of his kind, Mard belittles humans, and thinks of them as nothing more than insects. Indeed, he finds the mere thought of letting a human live when given the chance to kill them a tragedy, and is not above torturing his subordinates when they cause him any sort of inconvenience. He even this that his kind are also above the homunculi, and thinks that homunculi, as well as Soul Reapers and jinchuriki are just the same as humans are.
Despite his hatred towards humans, he isn't without mercy. He sparred Hashirama after defeating him and told the Hokage that the Allied Forces should fear the Demons and that they have no chance of winning. He even admitted that some of the Allied Forces managed to defeat four of his Demon forces, showing his more callous nature of not even caring about his own comrades.
However, as the remains of the Hell Legion is defeated and the only one left to take on Team Heroes aside from his master, Mard shows a sadistic, homicidal and cruel nature of himself, showing that he enjoys watching his opponents suffer, even if he was fighting children. He also insulted Fairy Tail and claimed them weak, and later reacts with anger when Naruto and Natsu talks back to him about the Demons being "idiots" for underestimating the Allied Forces and daring to step foot into the Hidden Leaf Village, sadistically attacking them with his Magic when Naruto and Natsu were already past the point of fighting back.
Hellfire Magic: Mard's signature magic that generates, manipulates and controls the dark, mystical properties of Fire Magic, or "hellfire" at will. This level of magic is what grants him the epithet "King of the Underworld", due to the usage of his hellfire powers.
- Hellfire Blast: Mard fires a straight beam of hellfire energy that explodes on contact, dealing serious burn damage to the target when hit.
- Hellfire Storm: Placing one of his hands forward, Mard fires a barrage of hellfire bullets onto a single or multiple opponents at once. This move could even tear through flesh when it on contact. Mard can also use this move in rapid succession.
- Hellfire Barrier: Mard generates a strong barrier made out of hellfire magic, which is capable of blocking any attack form the outside, but it can be broken if attacked from the inside.
- Hellfire Whip: Mard shoots out long, black and red, fire-like chains from his hands, complete with strange-looking grappling hooks at each end. These are capable of piercing through foes without actually hurting them despite the fire qualities, allowing him to grasp opponents and smash them around against surrounding objects.
- Hellfire Demon: A powerful spell in Mard's Hellfire Magic arsenal.
- Dark Flare: Mard fires a beam of dark energy towards his opponents, dealing burn damage on contact.
- Dark Gravity: Mard can use Darkness Magic by turning the dark qualities into gravity, weighing his opponent's body by a hundred ton, preventing them from being able to move for only three minutes.
Sensory Perception: Mard is an adapt sensor, able to sense the energy of any target he finds within a far reach.
Keen Intellect: Mard is very perceptive in combat and has a keen mind, able to deduce the mechanisms behind his opponents' attacks and come up with a strategy to counter-attack.
Master Hand-to-Hand Combatant: Mard has displayed incredible skill in unarmed combat, mostly using his punches, elbows and kicks to strike his opponents; even when using his magic with one hand, he still uses the other one free for close combat. He also uses either his Hellfire Magic or Darkness Magic to enhance his melee prowess, making his attacks far more dangerous.
Immense Strength: Despite his lean and young appearance, Mard is in fact an immensely strong individual, able to throw objects bigger than his size, and just with a single hand without effort.
Immense Durability: Mard is shown to be quite extremely durable, even with the most brutal assaults shown to be ineffective against him.
Immense Reflexes: Mard is an extremely agile combatant, having shown amazing reflexes and agility; he was able to easily counter an attempted attack by Hashirama while his back was turned, walking away.
Tremendous Magic Power: Mard possesses a tremendously amount of magic power that far exceeded his master, Grift Hades. His magic color is deep black with a dark violet outline.
- Enhanced Magic Power: The Devil's Eye further enhances Mard's already chaotic Magic power, creating a dark violet aura around him which keeps gradually increasing in magnitude, and which can terrify Team Heroes, including some of it's more advanced combatants.
- Black Arts: Thought the use of his Devil's Eye, Mard is able to perform one of Zeref's infamous Black Arts, which is written in the Books of Zeref that he read through Hades' teachings during his time in Tartarus.
- Living Magic: Through the activation of the Devil's Eye, Mard is capable of using Living Magic: a form of Magic that brings things to life under the master's control just like Grimoire Heart's Guild Master, Hades.
- Diablo: A more powerful Living Magic spell than Nemesis, as Mard is able to create numerous powerful Demons and Monsters from the debris of his destroyed airship.