Asuramaru is a high-ranking possession-type demon from the Black Demon series who resides within the Cursed Gear sword wielded by Yuichiro Hyakuya. Before becoming a demon, Asuramaru lived a past life as a human and later a vampire named Ashera Tepes. He was originally a noble born in ancient Greece during the dark ages, living happily with his mother and younger sister, Krul Tepes. After their home was destroyed and their mother killed during a revolt, Ashera was separated from his sister and sold into slavery. He was eventually bought by Sika Madu, also known as the first progenitor of vampires, who also held a slave named Yu who bore a striking resemblance to the modern-day Yuichiro. During this time, Ashera witnessed Yu transform into a powerful being known as the King of Salt. After being stabbed through the heart by this rampaging Yu, Ashera ingested some of Yu's blood and survived. Sika Madu later fed him more of Yu's blood, transforming him into a vampire and renaming him Asuramaru. Asuramaru came to address the first progenitor as father and was considered his favorite. Centuries later, he was passed into the care of Mahiru Hiragi, who wielded him as a demon weapon, before eventually being given to Guren Ichinose and then becoming Yuichiro's Cursed Gear.

As a demon, Asuramaru embodies a personality that is manipulative, arrogant, and bloodthirsty, thriving on feeding from his host's negative emotions like hatred and the desire for vengeance. He displays a strong sense of superiority, emphasizing his formidable power and the ease with which he could overpower his wielder. In line with typical demon behavior, he employs manipulative tactics, using haunting nightmares and traumatic visions to gradually seize control of his host's body. He often disturbs Yuichiro's dreams with visions of Mikaela Hyakuya questioning why he was abandoned, employing emotional and psychological torture. When assuming dominion over his host, Asuramaru treats the process as a strategic contest. Despite his cruel nature, he holds a deep disdain for humans, viewing them as a morally corrupt and greedy species that can be even more frightening than vampires or demons. He views seraphs with cautious apprehension, acknowledging their immense power and recognizing their potential to be even more dangerous than demons, going so far as to consider Yuichiro's contemplation of seraphic power a form of betrayal.

Asuramaru's primary motivation is to fully possess Yuichiro's body and consume his desires. He seeks to grow stronger by feeding on his host's power and emotions, and he has a singular intent to eventually take complete control. However, he also has an underlying goal connected to his lost past: to reunite with his sister, Krul Tepes, the queen of vampires. Despite his demonic nature and claims of having no emotions or memories, he retains a deep, buried care for his sister and seems to blame himself for cursing her with immortality. He is also shown to be following the orders of the first progenitor, Shikama Doji, working alongside other black demons like Kiseki-o and Gekkouin in a long-term plan to capture Mikaela's soul.

In the story, particularly during the Battle in Nagoya, Asuramaru serves as both a tremendous source of power and a constant threat to Yuichiro. As a Cursed Gear, he is essential to Yuichiro's combat effectiveness, granting him superhuman speed, strength, and agility. The initial stages of their relationship are defined by conflict, as Asuramaru tries to break Yuichiro's will during training. However, Yuichiro's unique response—refusing to give in and instead extending a hand of friendship and offering to become his comrade—shifts the dynamic. Asuramaru reluctantly accepts the contract and stops giving him nightmares, becoming a trusted adversary with a distinct purpose rather than a true ally. He provides Yuichiro with crucial information, such as revealing his non-human traits and discreetly alerting him to Mikaela's proximity. In desperate situations, Asuramaru fights alongside Yuichiro, and when Yuichiro is rendered unconscious or killed in battle, Asuramaru appears in a post-death state to help bring him back, imbued with even more demonic power.

Asuramaru's most significant relationship is with his wielder, Yuichiro Hyakuya. This relationship is a constant struggle of wills, but over time, Asuramaru is forced to become more rational towards him, stopping his most awful actions and even showing a begrudging respect for Yuichiro's determination to protect his family. He often refers to Yuichiro as a monster due to his relentless drive. Asuramaru also has a deep connection to Krul Tepes, who spent millennia searching for her lost brother. When Yuichiro meets Krul, Asuramaru becomes visibly agitated and later takes over Yuichiro's body to carry her in his arms, finally reuniting with her. Another key relationship is with Shikama Doji, the first progenitor, whom Asuramaru calls father. Though he treats him coldly and is annoyed when Shikama Doji enters his mind, he ultimately complies with his orders, such as when he is instructed to retrieve Mikaela's soul. He also has a history with other demons like Kiseki-o and Gekkouin, working with them on clandestine missions for the first progenitor, and he considers Noya Hienma, a demon he knew in his past life, to be a serious threat.

Asuramaru undergoes significant development as he slowly regains his memories of his past life as Ashera Tepes. Due to the effects of unique drugs administered to Yuichiro, who offered himself as a test subject, Asuramaru begins to remember fragments of his past, including his love for the sun and the sky, the existence of his sister, and his regrets about leaving her behind. He initially claims that demons do not have emotions, but his agitation upon seeing Krul tortured and his eventual tearful reunion with her contradict this assertion. His character evolves from a purely manipulative demon seeking only to possess his host to a more complex being who, while still dangerous, is driven by a millennia-old love for his sister and a complex allegiance to his father, the first progenitor.

Asuramaru possesses several notable abilities as a high-ranking black demon. His primary power is possession, allowing him to infiltrate and take over his wielder's body. He can change his form within the mental realm, appearing as different people from Yuichiro's past, such as a young Mikaela, to manipulate him. When his curse restraint loosens, horns grow on his head and his nails enlarge into claw-like structures. He can materialize his physical sword form for Yuichiro to wield, which enhances Yuichiro's combat abilities to superhuman levels. Asuramaru has a unique special ability called Ashura Kannon, which allows him to materialize and release a sword. When he has partially taken over Yuichiro, he can use techniques that distort dimensions, such as Mantou Unmei Kousaku. He is also capable of using Mugen Tou, which materializes only the scabbard and releases an invisible slash with the drawing command, as well as Mugen Battou Unmei Kousaku, which simultaneously stabs opponents with invisible swords. His extensive knowledge of curses, demons, and their weaknesses makes him an invaluable, if reluctant, source of information for his wielder.