He insists that he was born on June 6, at precisely 6:06 and 6 seconds, though his actual birthday is May 3. An eccentric oddball of an 11 year old, Souichi specializes in voodoo and enjoys causing havoc against people he doesn't like. Despite the dark nature of his actions, Souichi is shown to be childish, spelling things wrong or mistaking words, reminding us that he's still only 11. Souichi apparently suffers from anemia—as a result, he is almost always seen chewing iron nails in his mouth and sometimes makes them look like teeth. He dislikes his older brother Koichi and cousin Michina, and often tries to scare them and his sister Sayuri.
As an adult, he somehow masters high level of dark magic as he now can generate nails from his mouths and put a binding spell to turn his family into hideous figures to do his bidding in his haunted house business. At the same time, he marries Miss Fuchi and fathers a son who is a cannibalistic monster just like his mother. Due to his wife's terrifying personality, he has to live a nomadic life to get away from her as he goes from town to town opening haunted houses to get away from her. Eventually, his wife tracks him down and drags him away with her whilst threatening to kill him if he leaves her and their son again.
- Souichi tries to scare people often, sometimes using black magic, but his attempts almost always backfire on him in some way. For example, he hammers nails into dolls on trees, but the owner of the land gets angry and tries to kill him. As for the more mundane methods of scaring people, he tries to scare his sister with a spider only for his older brother to throw it back at him instead realizing that Souichi is also afraid of spiders.
- His black magic actually works on a few occasions; one example is when he causes a student to hallucinate and see a giant spider using a voodoo doll and toy spider.
- Souichi is shown to be rather unpopular and to be a loner in school, although he seems to think he is popular as shown when he mistakes girls whispering about how creepy he is and glancing at him as them admiring him.
- Souichi often talks and laughs to himself in school, making his classmates think he's super creepy.
- Souichi hates his cousin, Michina, who happens to share a birthday with him.
- Souichi only resembles one family member in the Tsujii family, his grandmother, who is dead now.
- Souichi took home a toad as a pet and to scare his sister with, but when he put it into a box with a lid as a make shift habitat he killed it accidentally.
- Souichi's grandmother claims that he had a twin named Souji; Souichi's parents say this isn't true, but Souji's ghost appeared along with his grandmother's in order to menace Michina. It is possible that Souji died in the womb before Souichi was born.