Atun is large humanoid demon with avian features. He has maroon feathers covering his body, dark purple skin, and yellow bird legs with black talons covering his arms, legs, shoulders, and sides. Atun also features a black bird-like decoration on his head with yellow eyes that have no pupils, large black wings, and black tail feathers. Atun's face features eyes with black irises and yellow sclera with a few markings near his cheeks.
When possessing Set's body, Atun possesses a muscular physique, diluted light blue eyes, red markings on his face, and black feather's covering the lower half of his body.
Atun like most demons has a dominating and cruel outlook with a large superiority complex, believing that he is stronger than anyone. He has a history with the demon Amon having a similar reputation as a mighty hero, but Atun wants to prove that he is stronger than him. He is willing to cooperate with a human for a chance to acquire more power, participating in Dr. Adams Cyborg Daemon project when it suits him, and then disposing the scientist.
Atun was a powerful demon said to be an equal to Amon, possessing equal strength, speed, agility. When brought together with Set as a Cyborg Daemon, Atun gained Set's psychic powers with Set's cybernetics enhancing Atun's already great powers including the Accelerator and self-repair functions. With the self repair functions disabled, Atun shows his natural abilities of flight using his wings. The many talons on his body can be used in several ways such as offensively shifting their positions including using the ones on his forearms as claw weapons, the tips of the talons can also extend to pierce through enemies. Atun can also channel dark colored energy either as destructive blasts or by sending them through his talons. The bird-like top can be used to track fast moving enemies, allowing Atun to attack them.
In the demon's past, Atun had met the demon hero Amon. However, never able to prove himself stronger, Atun bore a grudge against Amon. Many years after the demons reappeared on Earth, Atun came into contact with Dr. Adams. With the Cyborg Daemon project, Atun could grow more powerful than before. Atun agreed to the project and he was provided with the host Set, Cyborg 0018. However, 0018 was comatose from the experiments with Dr. Adams needing bait to wake up Atun. This came in the form of Devilman, Amon's body and power taken over by a human named Akira Fudo.
By the time Atun woke up in Set's body, Atun used Set's psychic powers to kill Dr. Adams having served his purpose. Atun shows his true form when he fights Devilman as he arrived to find Miki Makimura and later came into blows with 009. 0018's sister, Eva Maria Pallares/0015 tries to save her brother by deactivating the self-repair functions but this ended up fusing Atun and 0018 into a stronger form called Azazel. This form does much more damage, taking out the High-Teen Number Cyborgs.
Eva, with help from 001, manages to separate Set's soul from the gestalt, bringing Atun back into the front. Atun had then fought against 009 and Devilman, but Devilman wounded Atun and 009 was able to keep the energy blasts from harming Devilman. Catching Devilman with his talons, Atun attempts to finish him off by channeling energy directly into Devilman. 009 however performs a meteoric body slam on Atun, weakening him and giving Devilman the chance to incinerate him.
- Each of Atun's different forms make notable similarities with other characters from the Devilman Mythos, such as:
- In his first appearence, he takes the form of a fiery phantom-like state similar to the depiction of God from Demon Lord Dante.
- When first merged with Set, his appearence resembles Ryo Utsugi's humanoid demon form (also from Demon Lord Dante).
- In his full demon form, Atun ironically resembles Amon from the early quarter of his titular manga series.
- His resemblance to Amon could also be derived from God’s appearance in the 1972 anime.
- As Azazel, the angelic appearence is a nod to Satan.