Asuramaru is a high-ranked possession-type demon of the Black Demon Series. He formed a contract with Yuichiro Hyakuya and became his Cursed Gear in Seraph of the End: Vampire Reign.
He also appears in Seraph of the End: Guren Ichinose: Catastrophe at Sixteen as the cursed gear of Mahiru Hīragi.
Asuramaru appears as an androgynous adolescent male with long purple hair that goes beyond his feet. His bangs cover his left eye. As a vampire-gone-demon, he has red eyes, fangs, and pointed ears.
He wears a headband formed of two parallel golden strips that match the horns piercing through his hair from the edge of his hairline. The horns grow larger once loosens the curse restraining him.
He wears a sleeveless open-back dress including a collar with gold beads covering his throat. The chest piece is black while the rest of the dress is white. The gown splits open at his outer thighs, freeing his legs for movement. He wears black sleeves separated from the gown that reach his mid-upper arms and come down to his arms to his middle fingers, where each sleeve is attached to a ring. He is barefoot.
As Asuramaru exists as a mental concept, he can appear as anyone within Yuu's memories he chooses. He often takes the forms of the 12-year-old Mikaela Hyakuya as well as Akane Hyakuya, the other Hyakuya orphans, or even Yuichiro's parents. He chooses his forms depending on what he believes will be best for breaking Yuu's will.
Asuramaru is a blood-thirsty demon who was originally a vampire. He delights in drinking Yūichirō's blood and goes into a giddy high after Yuu feeds it to him for the first time. As a possession-type demon, he is described as particularly nasty and aggressive.
Like all demons, he is manipulative and uses nightmares to consume the one he possesses in order to take over his host's body. At the beginning of the series, he interrupts his host's dreams every night, making Yu tired during the day. After all, the only reason he allows Yuu to borrow his power is for the future opportunity to take over his body.
After Yuu loosens the curse on him and the two battle for dominance, Asuramaru eventually gives in to his compassion and submits to him. He also stops giving Yuu nightmares.
Even so, Asuramaru thinks of humans as vile, greedy creatures and says they are even worse than vampires and demons. He considers them to be taboo-breakers who will destroy the world. Although he is fully aware of Yuu going insane on the battlefield and of the experimentation done on him, Asuramaru does not reveal any of these vital secrets to his host. Asuramaru is more of a trusted enemy rather than a friend or ally.
Reason For Cross-Dressing
Asuramaru is naturally androgynous, looks young and feminine since he's originally a vampire. It's not stated why he dressed that way, but his appearance already resemble a young girl anyway. If there is a reason, it seems simply because vampire nobles had such taste in fashion.