The Father Spider Demon was, physically, the largest of his "Family". His skin was dark brown with the same red, dot-like design extending from his shoulders reaching down to his arms as other members of his family. Like his "wife" and "children," the Father Spider Demon also had long, pale white hair reaching down across his shoulders. His most unique trait was that his face resembled that of a gigantic spider with several eyes and spider pincers in both of his cheeks.
When he molted, he gained green skin with blade-like protrusions sticking out of his arms and legs. He lost all his hair and obtained sharper teeth and more pronounced mandibles. He also gained five spikes going upwards on his forehead. Finally, one of the most notable changes was that the number of his eyes doubled from nine, to eighteen.
The Father Spider Demon seemed to be the most devoted to his family as when he battled Tanjiro Kamado and Inosuke Hashibira, he repeatedly told them to stay away from his family while attempting to kill the duo. He appeared to be the most devoted to the role of the "Father" in Rui's fake family. However, he was also shown to be quite abusive towards his "wife," having repeatedly slashed and physically abused her in the past without even stating his reasons for doing so due to her being weaker than to what they originally expected of her.
After the Spider Demon (Daughter) calls for him, the Father Spider Demon lands in front of Tanjiro Kamado and Inosuke Hashibira. He angrily tells them to get away from his family, attacking them with a fierce punch. He easily blocks Tanjiro's Water Breathing, Second Form: Water Wheel technique, attempting to counter with another punch, only to be thwarted by Inosuke's twin swords pinning his arms down. He throws the two Demon Slayers off of him, immediately charging and attacking Tanjiro while repeating his earlier demand. Inosuke attempts to attack him from behind, only to be immediately punched. The Spider Demon continues his assault on the boar-headed Demon Hunter, only to be pinned by a tree that Tanjiro had cut. He quickly recovers from this, using the tree to smash his assailant into the distance.
Later, while Inosuke contemplates his next move, the Father Demon attacks him, breaking the tree he was leaning against. Surprised by his opponent's quick retreat into attack, the paternal Demon blocks the strike, but has his forearm severed when Inosuke uses both of his blades. Briefly examining his stump, the Father Demon then retreats, climbs up a tree, sheds his skin, and assumes a new form. He then descends the tree, ready to resume the battle.
Towering over his astounded opponent, the Father Demon punches him with great force, following through almost immediately with another blow, which Inosuke avoids by jumping into the air. Inosuke attempts to sever the Spider Demon's neck with his Beast Breathing, Third Fang: Devour, only for his blades to shatter upon contact. The Father Demon immediately counterattacks, sending his adversary hurtling into yet another tree, and then picks the Demon Slayer up by the throat. Readying another punch while simultaneously throttling his opponent, the Father Demon once again repeats his earlier demand, only to be stabbed in the throat by his resilient adversary. Unfazed, he tightens his grip on the youth's throat, but has his forearm severed once more, this time by the arrival of Giyu Tomioka. Howling in pain, the Father Demon immediately regrows it, charging at Giyu. His offense and existence are soon erased as Giyu strikes him with the Water Breathing, Fourth Form: Striking Tide, severing his neck and cutting him into multiple pieces.
Spider Physiology: Because Rui has shared his blood with him, the Father Spider Demon has gained several spider traits and abilities.
- Enhanced Strength: His most notable ability is his superhuman strength, being able to crush and lift entire trees and rocks with ease.
- Enhanced Durability: The Father spider demon has displayed incredible durability, being able to withstand attacks from both Tanjiro Kamado and Inosuke Hashibira.
Metamorphosis: The Father Spider Demon possessed the ability to shed his outer skin in order to transform into a more powerful form with increased physical power and regenerative abilities, making him even stronger and durable than in his original form.