Tsuchigomori

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Tsuchigomori (土籠) is a teacher at Kamome Academy. He is secretly a supernatural representing the academy's 5th School Mystery, the 4PM Bookstacks, and also its curator.

Appearance
Tsuchigomori is a tall, lanky man. He has spiky black hair with long white fringes and purple eyes.

When he disguises himself as a human, he looks almost indistinguishable from any regular person, while in his true supernatural form he has noticeably sharper features such as sharp ears, claw-like hands and shark-like teeth. He also has 4 additional limbs protruding from his lower back.

He wears a white coat with a purple sweater and a golden eyeglasses.

In 1969, he is shown to have all-black hair and doesn't wear glasses, his sweater is also replaced with a dress shirt.

Personality
While Tsuchigomori seems like a normal, caring and sort-of-meek teacher in his disguise, he is actually sarcastic, mischievous and quite short-tempered. He's also shown to be quite sadistic, having fun in threatening and scaring Nene and Kou ― although he quickly stops following a light warning from Hanako.

Although he is a teacher, he is shown to dislike children because of their noisy attitude. Tsuchigomori also likes to joke around and even with his odd sense of humor, such as when he joked about liking to eat human children, but only receiving terrified looks as a response.

Despite him saying that he only pretends to care about his students, being a supernatural that can only pretend to be human, he has been shown to actually care for them genuinely.

When asked why his teacher persona is different than his actual personality as a supernatural, he answers that it's just because he feels like it.

Hanako
Tsuchigomori is one of the supernaturals who had a past history with Hanako. Tsuchigomori has been teaching in Kamome Gakuen since a long time ago, living as a supernatural in the school. He was Hanako's teacher and seems to care a great deal about him. He was deeply impacted by Hanako's ability to change his pre-determined future, resulting in the No.5's yorishiro being the lunar rock that he received from Hanako.

After Hanako takes on the role as School Mystery Number 7, he effectively gains command over the School Mysteries, including Tsuchigomori, as a leader. They have a friendly relationship due to their shared past, though Hanako can intimidate Tsuchigomori into following his orders.

Yashiro Nene
Nene and Tsuchigomori didn't have a lot of interactions prior to the reveal Tsuchigomori's true identity. Being a curator of the 4PM Bookstacks however, he knows about her lifespan and predicted future.

Tsuchigomori seems to care about Nene as a student, and trusts her enough to tell her about his time as Hanako's teacher after she saw glimpses of it through destroying his yorishiro. He tells Nene that it's up to her what she does with the information, and to rely on him because after all, he is still her teacher.

Minamoto Kou
Tsuchigomori is Kou's homeroom teacher. Kou was surprised by the reveal of Tsuchigomori being a supernatural disguising as a teacher. Though Tsuchigomori notes it is not uncommon for supernatural beings to adopt disguise to maintain their connection to the Near Shore.

Tsuchigomori seems to be annoyed at Kou at times, due to Kou referring to him as "Spider-face", and trying to feed him bugs once. Despite this, he still cares for Kou, allowing him to take a peek at Nene's book despite his insistence that it's impossible to change the future that's already written.

Yako
Tsuchigomori and Yako have been shown bickering ever since his introduction as supernatural. He derogatorily calls her "stupid fox" or "echinococcus" while being addressed as "emo spider" as well as getting bitten by her in her fox form.

Despite their constant quarreling, they both are shown regularly interacting with each other in contrast to the rest of the seven school mysteries who keep to themselves.

Number 5's Powers
As the curator of the 4PM Bookstacks, he has access to the entire body of knowledge surrounding the school, of the present as well as past and future. He seems to know everything about every student that has ever attended the school.

Even after his yorishiro and boundary were broken, his powers still allow him ways to retrieve the books in the library.

He mentions that because of his position, it's possible for him to alter the future, but does not have any plans on doing so because any curator who does will be erased from existence.

Trivia

 * His favorite snack is bekko candy.
 * His latest concern is how hard it is to live a double life as a teacher and a supernatural.