Sōichi Tsujii is an eccentric, gaunt 11-year-old boy who constantly gnaws on iron nails to combat his anemia. He falsely declares his birthday as June 6 at 6:06:66, though he was actually born on May 3. His reclusive and antisocial nature leads him to talk to himself and laugh alone, earning him a reputation as a "creep" among classmates who mistake their attention for admiration. He shares a resemblance to his deceased grandmother, who claimed he had a twin brother named Souji who died in the womb – a story his parents deny. Sōichi harbors resentment towards his older brother Koichi and frequently torments his sister Sayuri and cousin Michina with pranks. Sōichi practices onmyōdō and black magic, specializing in using katashiro (voodoo dolls) to inflict curses, hallucinations, or physical harm over unlimited distances, such as hospitalizing victims by hammering nails into their corresponding dolls. His powers include generating and shooting nails from his mouth, reanimating corpses, manipulating weather, creating sentient human-sized dolls, and constructing soundproof rooms – which he exploits to torment others with insect noises. He performs rituals like "Ushi no toki mairi," wearing candles on his head and nailing straw dolls to trees. His malicious schemes often backfire: a spider meant to scare Sayuri terrifies him when Koichi throws it back; nailing dolls angers a landowner who tries to kill him; a pet toad dies accidentally. Rare moments of sympathy emerge, like halting a ghostly attack on Michina after she saves him cake on their shared birthday. He occasionally interferes in others' conflicts through disturbing curses. As an adult, Sōichi masters advanced dark magic, generating mouth-nails and enslaving his family via binding spells to operate a haunted house attraction. He marries Miss Fuchi, a cannibalistic entity, and fathers a monstrous son, Binzo. Adopting a nomadic life to evade Fuchi, he establishes haunted houses in new towns until Fuchi tracks him down, threatening death if he abandons her and Binzo again. In one incident, his adult self enslaves Koichi, Sayuri, and his parents in the haunted house, causing them to age rapidly due to malnourishment. Binzo nearly devours Sōichi during a confrontation before Fuchi intervenes and drags him away. Key childhood stories include: "Fun Summer Vacation," where he immediately curses cousins Michina and Yusuke after they discover his nail-chewing; "Fun Winter Vacation," where he bites Yusuke's classmate Yumiko, causing illness, and harasses her during recovery; "Souichi's Diary of Delights," revealing detailed curses via his discovered diary, including causing Yusuke's accident; "Souichi's Birthday," highlighting family neglect on his birthday in favor of Michina's, prompting him to summon his grandmother's ghost and imaginary twin brother to terrorize her – a plan abandoned after she leaves him cake and a card; and "Soichi's Beloved Pet," where he corrupts a stray kitten into a bakeneko that attacks insects and unleashes lightning. Adult-focused stories "Secret of the Haunted Mansion" and "The Souichi Front" depict his haunted house as a lethal trap where customers risk being eaten by Binzo. When Michina investigates the family's disappearance, she finds them aged and emaciated under Sōichi's control; he attempts to feed her to Binzo, but Binzo turns on him instead.