Before we begin with the actual theorizing, I wanted to show you my analysis of the opening song, as I'm going to bring it up quite a bit in my theories. I understand this isn't the full opening, but I can't seem to find an English translation of the full version yet. Let me know if you find one!
Bold - lyrics
Normal - my analysis
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(Hanako-san of the toilet)
Now then, let's talk, No.7. (Number seven)
Eh? Hah? One joy, one grief, the past and the future
This is most likely referring to Hanako's backstory, as we know he was being abused by someone, most likely Tsukasa. The "one grief" is Hanako's, as he's being abused, and the joy is Tsukasa's, who loves hurting his brother because he's a psychopath.
(Hey hey) On the ghost story, circulates the shadow an identical two
The "ghost story" is probably the rumor about Hanako-san of the Toilets, and the "shadow of an identical two" is Hanako and Tsukasa as they are identical twins.
(A secret, no expectations, just a story)
The secret is Hanako's past. The story is the rumors circulating the school. What's interesting is the part where it says "just a story" when we know how rumors can influence the supernaturals.
A sweet fruit mixed with unreality
The "sweet fruit" may be referring to Yashiro, as she is constantly being compared to a daikon radish, which are known to be sweet (at least according to Google). She is "mixed with unreality" as she is involved with the supernaturals.
Right right, I've told you three times and over.
This could be referring to how you have to knock three times to summon Hanako-san, though I'm not entirely sure about this one.
(See see) Four kinds of suffering and torment, You're not listening at all aren't you?
Maybe Tsukasa subjected Hanako to four different types of abuse/pain?
(A secret, no expectations, a never ending story)
in the end a rumor, a delusion of reality
After all of the abuse, Hanako and Tsukasa died. Afterwards, rumors about a ghost haunting the girl's bathroom started, and Hanako became the 7th mystery.
A fabricated picture like ten winds and five rains of a peaceful world, morning noon and night
The part about the fabricated picture could be referring to various different things. It may be a reference to the Picture Perfect Arc, where Kou and Yashiro were stuck in the fake world. It might also be talking a real fabricated picture or even a fake image in one's mind. I don't know what the rest could mean, though the part about a "peaceful world" might tie in with the possibility of this being a nod to the Picture Perfect Arc.
Purifying the six sensors is foolish, it won't grow, grass on the scythe
This might be part of how Hanako said that God told him if he could fulfill his duties as No.7 his sins would be forgiven. Part of his job might be "purifying the six sensors". What the "six sensors" are, I'm not sure.
I want some magic
My seven faults are incomplete
This probably also involves God forgiving Hanako's sins, as mentioned before. His seven faults could be the wrongs that he may have done in life, though other than killing his brother, it doesn't look like he did anything wrong.
It's unfinished, so I'm struggling now
Maybe Hanako hasn't finished doing his job, being No.7 of the Seven Mysteries, and he's struggling to do so.
Words all over the place, a blade over your head
The "blade over your head" is definitely the knife Hanako used to kill Tsukasa.
Wearing a smiling mask of lies
The "smiling mask of lies" might be how Hanako lies and keeps secrets about his past and Yashiro's lifespan and does it all while smiling.
(You're alone) Getting hurt for someone
This part is talking about how Hanako was being abused by Tsukasa, as Hanako was being hurt for Tsukasa's pleasure.
The gap between sin and punishment
Perhaps this is referring to Hanako atoning to his sins. Maybe he sees atoning for his sins as punishment.
In that place with no one there
I'm sure he's there
The "place with no one there" is probably Hanako's boundary: the girls' bathroom. But since there's no water on the ground (one of the most obvious signs of a boundary), then Hanako's boundary might actually be the stall where you summon him from. The "he" might be Tsukasa.
Now then, let's talk, No.7. (Number seven)
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That was my analysis of "No.7"! I hope you enjoyed and if you have your own ideas on what this song could mean, then tell me about them in the comments! I'd love to theorize with you.
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Toilet Bound Hanako-kun Theories
RandomRead the title. {POSSIBLE MANGA SPOILERS AHEAD} (p̶l̶e̶a̶s̶e̶ ̶r̶e̶a̶d̶/̶w̶a̶t̶c̶h̶ ̶t̶o̶i̶l̶e̶t̶ ̶b̶o̶u̶n̶d̶ ̶h̶a̶n̶a̶k̶o̶-̶k̶u̶n̶ ̶i̶ ̶n̶e̶e̶d̶ ̶m̶o̶r̶e̶ ̶p̶e̶o̶p̶l̶e̶ ̶t̶o̶ ̶t̶a̶l̶k̶ ̶t̶o̶)
