|“||I'm going to be the Magic Emperor! This is something I'll never give up on!||”|
— To himself in The Boy's Vow
Asta is a young man of short stature with a muscular build. He has green eyes and messy ash blond hair with a single strand protruding upwards from the center of his head. He keeps his hair together with a black headband, which has the gold-colored Black Bull insignia and has a red-colored four-sided star with three stitches on the back of it.
Asta's common outfit consists of a plain-white tunic with a V-shaped collar underneath a dark blue jacket whose sleeves extend only up to his elbows. He sports a pair of matching shorts which has a stitch marks on the left side. His pants also extends just below his knees. His shoes is made out of two different cloths, the parts covering his ankles, the soles the same color as his jacket and pants. The parts covering his toes are white, a vertical stitch mark going from the bridge to his toes. When he's sending his anti-magic in his body, it surrounds the right part of his body with black matter and he gains a partial demon transformation.
After receiving his grimoire, Asta wears a pouch strapped to a brown leather belt. Additionally, as a member of Black Bull, he wears his squad's signature robe, which is black and short with gold trimming and covers the upper half of his torso. It also has a hood and a gold-colored button located at the right-hand side to hold it together, while the left-hand side displays the squad's insignia.
Asta generally is cheerful and hyperactive. He tends to shout to express his thoughts and expresses his goals, regardless of other people's opinions, as seen when he expressed his goal on becoming a Magic Emperor during the Magic Knights entrance exam, where he tells other people who disagree with him to be quiet. His strong tenacity interests Yami Sukehiro enough to recruit him into his ranks. His strong determination also leads him to have a strong will to never give up and keep standing during a harsh battle, as acknowledged by Magna Swing.
Because of his upbringing at the Forsaken region, Asta frequently receives harsh opinions and insults from almost everyone he meets. This makes Asta have a slightly sarcastic nature as he tries to defend himself. But, having received harsh opinions from others, Asta has developed an immunity to them as he could easily brush them off without getting overly annoyed. However, Asta is still annoyed when others deeply insult his comrades; he will not hesitate to confront them as he tries to defend his friends.
Asta also has a very strong sense of justice, which is shown during his fight against Heath Grice, as he believes that everyone deserves to be protected. It is also not limited to only his comrades but his enemies as well, as noted when he asks his fellow squadmates to take the unconscious Mars along with them while the dungeon is collapsing. He argues that their mission is only to explore a dungeon as they are not ordered to kill any enemies. Therefore, Asta seems to see the positive side of everyone and never takes an insult too seriously, as seen when he still befriends both Yuno and Noelle Silva even after both given him harsh opinions.
- Anti Magic: Asta uses this form of magic to nullify other magic. After being healed by the Witch Queen, Asta can channel Anti Magic through his body faster and in a more condensed form. During the Royal Knights arc, Asta can only enter his Black Form twice a day.
- Expert Swordsman: Asta displays the ability to wield a sword, although still only at a level in which he swings it only in his opponent's direction and with the addition of his second sword's different shape and abilities, his tactical usage of his swords has improved. After having been trained by Fanzell Kruger, Asta's swordsmanship has improved.
- Enhanced Strength: Asta possesses an above-average physical strength, able to do a thousand single-handed handstand push-ups, to swing his sword single-handedly, and to punch holes in rock walls with no injury to himself. Asta can swing his swords with inhuman force, able to damage powerful magic-wielding opponents.
- Enhanced Speed: Asta is capable of moving in an above-average speed, which causes William Vangeance to think that he has used magic to aid himself.
- Enhanced Durability: Asta possesses a high durability, as seen when he is able to keep standing up, despite receiving numerous attacks from Heath. He is even able to emerge from Licht's attempted cave-in relatively unharmed alongside Yami.
- Enhanced Instinct: Asta possesses a heightened level of instinct, as demonstrated when he can unconsciously moves his body in the best way to defend himself.
- Ki: A principle originating from Yami's homeland, this allows Asta to use a sixth sense and to perceive and react to unseen attacks and persons.
- Grimoire: Asta wields a five-leaf clover grimoire, which he receives during his fight with Lebuty. The grimoire is tattered and covered in dirt, a black five-leaf clover insignia at the center of its front cover. This clover symbol is largely unseen due to the dirt covering it, making most (including Asta) assume that the grimoire possesses unusual magic but is otherwise ordinary. Asta is able to summon a sword from it, which he uses for his anti magic. His grimoire appears to be very unique, as Licht is rather annoyed that someone like Asta possesses it. Licht also claims that the grimoire actually does not belong to Asta, but to someone else.
- Demon-Slayer Sword: Asta's original sword can cut spells with its edges and deflect them with the flat of the blade.
- Demon-Dweller Sword: Asta's second sword can also cut spells with its edges and can borrow the magic from his allies, thereby increasing the force and range of his attacks. Only Asta can wield this sword because he possesses no mana, as other mages would not be able to use any magic while in contact with the sword and would have their mana reserves depleted.
- "I'm going to be the Magic Emperor!! This is something I'll never give up on!!" 「オレは 魔法帝に なる...!! これだけは 譲れねー...!! "Ore wa Mahōtei ni naru...!! Kore dake wa yuzurene...!!"」
- "Even with no magical powers... I'll become the Magic Emperor." 「魔力が 無くても オレは 魔法帝に なる "Maryoku ga nakute mo Ore wa Mahōtei ni naru"」
- "My magic is never giving up!!!!" 「諦めないのがオレの魔法だ!!!! "Akiramenai no ga ore no mahōda!!!!"」
- "I may be just a pebble, but a I'm the pebble that can shatter diamond!!!!" 「石コロは石コロでもオレはァア ダイヤモンドを砕く石コロだ!!!! "Ishikoro wa ishikoro demo ore wa~aa daiyamondo o kudaku ishikoro da!!!!"」
- Asta's most favorite person is the sister who took care of him while he was living in Hage.
- Asta is the eleventh smartest Black Bull.
- Asta is the second most muscular character.
- Asta ranked first in the first popularity poll.