The Older Brother Spider Demon was a member of the Spider Family.
As Agatsuma Zenitsu 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 Hunter 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.