Spoilers for the film, the Siren arc, and the fifth theater bonus booklet follow.
A reasonable prediction was: “People are not exploding over this secret booklet, so maybe the secret is not that huge.”
That was half right.
The fifth admission bonus for Chiikawa the Movie: The Secret of Mermaid Island, distributed from September 19, 2026, is a 52-page booklet. Its publicly confirmed pillars are early story illustrations from the Siren arc, a newly drawn food-essay manga titled Mucha-Uma Diary, and the special manga Ode and the Island, in which Ode and Shimajiro get into a physical fight [1][2].
So no, this is not:
“The Siren was secretly X!” “Hitoha and Futaba rewrite the entire cosmology!” “The hidden law of the Chiikawa universe has been revealed!”
The real treasure is more creative-process nerd than lore theorist.
It is a deletion log.
1. The biggest secret: Hitoha and Futaba originally had visible faces and dialogue
In the official interview, Nagano explained that early drafts gave Hitoha and Futaba both faces and lines [3].
Before the Siren’s design was finalized, “Ano Ko” was also used as a placeholder while the story was being planned [3].
That makes the finished version look radically different.
In the final work, we do not see Hitoha and Futaba’s faces directly. We do not hear their dialogue. We infer what they think from movement, distance, timing, and consequences.
The early version offered more interpretation for us.
Faces. Speech. Expressions. More obvious emotional cues.
The path to the final version was therefore not “add more character.”
It was remove information aggressively.
2. “Just make them mobs” is not a Nagano quote—but it is a brutally efficient joke-summary
One reaction is basically:
“So the edit was: eh, make them mobs.”
Important distinction: Nagano did not say this.
The official explanation is that the two were talking too much in the rough version, and that Nagano preferred them without lines, leading to the final form where their faces and speech are not directly available to the audience [3].
So they were not actually demoted into disposable extras.
A better description is:
The characters stayed. The explanations disappeared.
The film still treats them as distinct individuals. Nagano describes Hitoha as slightly airheaded and Futaba as more dependable, and the casting process specifically looked for performers who could preserve those differences even without dialogue [3].
They are not mobs.
They are major characters wearing the information density of mobs.
3. Why does removing a face and dialogue make them creepier?
A face answers questions fast.
Regret. Fear. Relief. Calculation. Guilt.
Dialogue answers even more.
“I was thinking this.” “I had no choice.” “I wanted that.”
Remove both, and the work gets outsourced to the audience.
Was that kindness? Self-preservation? Planning? Panic? Did the two agree with each other? Did one follow the other?
The answer never fully locks.
That keeps Hitoha and Futaba from settling into a single moral category such as “poor victims” or “obvious villains.”
Missing information becomes an empty seat for interpretation.
This is where the “make them mobs” joke becomes strangely accurate.
Usually, reducing a character’s information makes them weaker.
Here, reducing information makes them more memorable.
Less is more.
Less explanation, more residue.
4. Not explaining them also protects the moral grayness of the Siren arc
The Siren arc is difficult to compress into “this one person is the bad guy.”
If Hitoha and Futaba explained all their feelings, the audience would be pushed toward a stable reading.
If they cried and justified everything, sympathy would rise. If they explained a scheme with an evil expression, they would read as villains. If they explicitly apologized, the story would tell us how to classify them.
The final version does not give us that convenience.
It leaves actions.
So the audience keeps arguing internally:
“That was still wrong.” “But if you were cornered that badly…” “No, wait, even then…”
The characters say less, so the viewer’s internal meeting lasts longer.
That unresolved quality is part of why the arc lingers.
5. “The booklet probably isn’t a huge deal” was half correct and half wrong
Return to the original prediction.
“If the release did not trigger a giant lore explosion, there probably is not a world-breaking revelation inside.”
Fair.
As of September 20, 2026, the publicly verifiable highlights are primarily production materials: early concepts, unused scenes, newly drawn side manga, and an essay [1][2][4].
If you wanted a book that dumps dozens of new canonical truths, the blast radius is modest.
But if you want to see how the finished work was created by subtraction, it is excellent.
One publicly shown page even discusses a scene Nagano considered drawing—quietly carrying someone home—but ultimately left out [4].
Finished works look inevitable after the fact.
They were not.
There were faces. There was dialogue. There were placeholders. There were alternate scenes. Then the knife came out.
Creation is not only addition.
It is also accumulated deletion.
6. The best secret is not what was added, but what was erased
A “secret book” sounds like a container for new lore.
Here, the most interesting material is the opposite.
Not a document where Hitoha and Futaba gained faces.
A document from before the faces were removed.
Not new dialogue.
Evidence from before the dialogue was removed.
Not only the finished Siren.
A glimpse of the stage where the Siren did not fully exist yet.
This is less a lore bible than a Git diff.
Final: “Two silent, faceless figures you cannot stop thinking about.”
Early: “Two visibly expressive characters who talk more and are easier to read.”
Diff: face -1 dialogue -a lot uncanniness +a lot interpretive space +a lot
7. Conclusion: “make them mobs” is inaccurate, but weirdly close to the creative truth
They were never simply mobs.
Their personalities remained carefully differentiated, and the film even cast them with nonverbal characterization in mind [3].
But the direction of the edit can be summarized brutally:
“Stop explaining them. Pull back the faces. Pull back the dialogue.”
The result was a shift from understandable characters into characters the audience keeps trying to understand.
The fifth booklet does not appear, in its publicly confirmed contents, to contain a universe-shattering lore bomb.
It contains something arguably more useful for understanding why the finished story works:
the record of what was removed.
The secret was not the lore.
The secret was the deletion history.
Sources
- 電撃オンライン「映画『ちいかわ』入場者特典第5弾は“ひみつ本”。ヒトハとフタバに顔があった、セイレーン編初期構想などが掲載」(2026-09-14) dengekionline.com
- シネマトゥデイ「『映画ちいかわ』入場者特典5弾は全52Pの『ひみつ本』 9月19日より配布」(2026-09-14) cinematoday.jp
- 『映画ちいかわ 人魚の島のひみつ』原作・脚本：ナガノ オフィシャルインタビュー chiikawa.toho-movie.jp
- ORICON NEWS「『ちいかわ』特典本の新たな中身ページ解禁でネットざわつく『これ見せていいんですか！？』」(2026-09-17) oricon.co.jp
