The older brother Spider Demon body takes the form of a giant spider possessing a human head, his face similar to the Mother and Older sister demon and has the same facial features of red dots, pure white skin, and short white hair similar to his "younger brother" Rui, with the only difference being that his eyes possess dark red, almost black Sclera with deep blue irises and pupils.
The Older Brother Spider demon is shown to be sadistic and cruel, that in the flashbacks of the Mother Spider Demon he openly taunts and enjoys the physical and verbal abuse she was being put through by Rui and the Spider Demon Father, while sarcastically calling them "Mother" and "Father", showing that he sadistically enjoys playing the part of being in a "Family".
At one time the Older Brother Spider demon became a demon and took up refuge in Natagumo Mountain. When Rui came to the mountain he chose the Older Brother Spider Demon to be his older brother, having him modify his own body to fit in.
As Zenitsu Agatsuma panicked at the sight of various humans in the process of becoming spiders, the Older Brother Spider Demon descended from his house, causing the terrified Demon Slayer to flee. The Brother Demon warned the youth that he had already lost, telling him to look at his hand. He noted that the Demon Hunter had been bitten by a spider and poisoned, and would become a spider himself in half an hour's time. Taking out a watch, the Brother Demon explained the various effects the poison would have to his opponent, as well as the times they would take place at. Zenitsu freaked out at this knowledge, running and climbing up a tree, for which the Spider Demon both chided and questioned his decisions. He went on to tell the youth that he had nothing to fear, for he would lose his intellect upon transformation, only to be irritated by the boy's terrified, brutally honest comments.
As Zenitsu continued to complain, the Brother Demon was bemused, and was similarly confused as the youth fainted and fell out of the tree. However, the Demon Hunter quickly recovered, charging at the Brother Demon, who spewed acid-like venom at his aggressor. Zenitsu dodged this attack mid-air, then landed on the ground, while the Spider Demon contemplated this new change in circumstance. He ordered his spider henchmen to attack Zenitsu as the youth prepared another attack, which was dodged, prompting the Brother Demon to spew venom at the Demon Hunter again. Although this too was dodged, the Brother Demon confirmed that Zenitsu could only use one technique, prompting him to gloat at his apparent victory.
The Spider Demon continued his assault with yet another spewing of venom, this time managing to strike Zenitsu, while his henchmen attacked the weakening youth. He ordered them to envenom the Demon Hunter, which was dodged, but he noted that the overexertion was causing his opponent to vomit blood, which prompted him to label Zenitsu as useless to him. However, the youth prepared to resume his attack, while a surprised Brother Demon noted the air was shaking. He retreated back into his house, letting loose another spray of venom, but Zenitsu's enhanced Thunder-Clap Flash technique cut through him, severing his neck. The Brother Demon's last moments were of utter disbelief that Zenitsu had defeated him, his body hitting the ground in two parts.
Abilities and Powers
- Spider Conversion - The Older Brother Spider Demon reveals that one of his venom's possesses the ability to convert a human being into a spider like creature, with the only human trait remaining in them being their human heads, turning them into the same spider-human hybrid like himself, completely losing all intelligence and becoming subservient to him. The process of transformation itself requires at least 30 minutes for the victim to be completely transformed.