Although there are plenty of overlapping elements, I’ll consider the thought route for deducing the culprit and the closed room separately.
The culprit is…
(Idea shamelessly stolen from here)
Deducing the culprit was pretty simple, really. Whenever and however they did it, it was clear that the culprit got their hands on the gold. The most reasonable assumption is that they hid it somewhere on the island. However, most of the family was due to leave the island the very next day. There was no way anyone in this group would be able to smuggle a half-ton of gold off Rokkenjima. The only way that someone in the branch family group would reap any of the benefits of their theft would be to come back to Rokkenjima later. A huge risk, since even if the gold evaded the police investigation (not hard to imagine, it’s a big island after all), it’s unlikely they could stop Krauss and his family from tearing the island apart looking for it in the first place.
So, we’re left with only three likely suspects: Krauss, Natsuhi, and Jessica.
And before we get into the “prior knowledge not required” argument, do note that the narrative it’s stated that “The other (not Natsuhi) branch family members, who were simply visiting the island of Rokkenjima anyway…”. We could also get into the possibility of just Kinzo’s children being considered the “main” family living on Rokkenjima, but you can see why that’s unreasonably strange, no?
The culprit also did something else during the family conference - they planted a threatening letter addressed to the family, demanding they never return to Rokkenjima. The fact that they were trying so hard to scare them away is further evidence of the main family’s guilt. After all, they are the only ones who would be “forced” to stay on the island despite the appearance of the witch. And even if they planned to leave Rokkenjima for a time, they would have the most cause to return to their home at some point, allowing the culprit to retrieve the gold at their leisure.
So, about that troublesome vase… as stated in red, it was broken by the culprit themselves. However, Natsuhi has a perfect alibi for that time period, as she spent 6 AM to 7 AM with Kyrie.
That leaves Jessica or Krauss. It’s not proven beyond a shadow of the doubt in the narrative, but Krauss is the most reasonable conclusion. And with the red truths concerning the number of people in the mansion that morning, Jessica becomes completely impossible.
Incidentally, this is pretty much all that Kyrie figured out. Once the gold was confirmed to be missing, she was able to deduce that only a handful of people would attempt the scheme at all, and one of them had an alibi that she could confirm herself. All that follows was a mystery even to her, but her next step would probably have been to try to lift the box once more. It would all unravel for Krauss from there.
The Locked Room Loop
Herein lies the complexity. The culprit’s goal was simply to confuse the others with an impossible problem, hoping to plant the seed of some outside culprit, the witch “Beatrice”, being responsible. Whether they believed in the magical explanation of the key paradox, or simply believed some third party was the only explanation for the gold disappearing under their guard, he’d hope to cause chaos with this gambit.
We’ll start with the easier puzzle, the disappearance of the gold. As many of you reasoned, unless a thief X was at play, no member of the family could have stolen the gold after the discovery of the body. But… the box was found to still be immovably heavy shortly after that, and thus it was concluded that the gold was still in place. How is this possible?
Both teams were able to get this far. The gold could not have still been in the box after they discovered the body. However, the box was indeed unmoveable, as confirmed by three different people (Eva’s relief can be explained by her testing it quickly before the group’s arrival, as she ran in ahead. This isn’t really an important detail at all, however.) It’s thus reasonable to conclude that the box was glued in place. After all, its weight was never tested after the gold was found missing.
And the clue to the existence of a strong adhesive on Rokkenjima? Genji repairing the broken chair, of course. The chair is described as looking just like the others, so fixing it with nails is unlikely. The glue is established to be strong, as Battler basically goes and tries to break it again, but isn’t able to. An important detail to note about this glue is that it takes a good amount of time to set. This is why Genji couldn’t unveil the chair until the next morning.
So the gold had been missing since way before Genji woke up, and the box had been glued in place. Let’s keep this in mind for later.
As established by the red truth, the servants’ keyring was not in its rightful place when the conference began. It is reasonable to conclude that Krauss had taken it before the conference, planning on killing Kinzo during the conference. Perhaps he intended to forge his will, expecting that Kinzo would be leaving them nothing as things stood. If he had to, he could bribe his siblings into silence over this strangely generous will appearing out of thin air. By this point, he probably didn’t have a particular scheme in mind, though. (Or, Krauss just wanted to sneak a peak at the will, because unlike his siblings he knew that they were in poor standing with their father)
And if we’re getting even more nit-picky, why not steal the master key? Simple, Krauss wouldn’t have had the ability to. This part wasn’t necessary, but the narrative does state that Genji was sure the master key never left his possession. This can be explained by him keeping said key in a combination locker in the servants’ room, a code only he would know. Again, this was completely unnecessary to state. The only key takeaway is that Genji was sure nobody took the master key during the conference.
Fast forward to after the meeting with Kinzo. Things have become both simpler and more dire for Krauss: forging a will is probably insufficient, now that his father has claimed that the rest of the inheritance is “out of their reach”. He doesn’t know exactly what this means, but it’s safe to assume its gone. However, a much more direct approach has presented itself: the cache of gold. Even more fortunately, the families are being split up across the guesthouse and mansion, leaving him free to act that night. He’s also spotted a few crucial details (one of which being the glue used to fix the chair, as he was present in the dining hall during that opening scene) about the scenario in general, which will become important later.
Let’s lay out the important facts. All of these could have been confirmed by repetition requests, or you could have taken the narrative at face value.
- The key found in the top drawer was definitely the key to the storage room. It wasn’t swapped at any point.
- The key found on Kinzo was indeed the key to the top drawer. It was not swapped at any point, and was present on Kinzo’s body before Genji unlocked the storage room for the first time.
- The top drawer was locked.
- The storage room door was locked.
- The storage room’s window was closed and locked from the moment Genji entered the storage room for the first time, throughout the entire rest of the incident.
So, it’s impossible, right?
Not quite. It all becomes clear if we assume the culprit used one more tool… our old, dear friend…
Fishing line, to be precise.
The mechanism itself is rather complex, and required looking at the floorplans. As you may have noted, the windows of the two rooms in question line up pretty neatly. This is why Kinzo’s body was moved to the storage room in particular: Kinzo’s body needed to be found here (and it needed to be unclear whether he was already dead, but more on that later) to ensure that the culprit could escape. It was also important that this was Genji’s routine first stop, a fact that Krauss would know well.
First, Kinzo is killed in his study with one of his decorative stakes. He takes both of the box keys as well. Krauss drags him to the storage room, and locks the door, keeping the key. He’ll return later.
Next, he clears out the box upstairs, stashing the gold somewhere safe. Where is unimportant, which I think everyone agreed with.
The box is now empty, and thus light enough to move around easily. This is where things get a bit ridiculous. In short, using fishing line and the box’s handles, along with two pivot points in the VIP room, Krauss was able to remotely return the box to its resting place. Krauss can set all this up from the VIP room window. It is, after all, quite large. If a person could easily fit through, we can assume the box can as well (that was the point of the detail regarding the number of ingots that could fit in the box).
Ignoring string B for the time being, string A is essentially a pulley system (using the curtain rod as the pivot point) that allows Krauss to lift the box back up to the VIP room window, from the storage room. So assume he sets this mess up in the VIP room, leaving the window open. He returns to the storage room and locks the door behind him. He pulls string A until the box is level with the storage room window. Through the window, he puts the storage room key in the top drawer, then locks it. He continues pulling string A until the box is level with the VIP room window.
Next, he must rotate the box so that it will be pulled into the proper orientation. He does this by pulling on string B while gradually letting go of string A, allowing a 90 degree rotation level with the window. He can verify that he’s succeeded by sticking his head out the window as he does all this, by the way:
He then continues pulling string B (gradually giving string A more slack) until the box is back in place, right on top of a patch of glue he left in the VIP room:
All that’s left is to undo the loops (see the closeup above, each string A/B is actually a doubled up length of fishing line, so once he’s done with all this he can just pull on one end to retrieve the line).
Whew, okay. Just a note, I didn’t expect anywhere this level of detail for a blue truth.
Now he’s free to lock the storage room window from the inside, put the top compartment’s key on Kinzo, and wait. He spends the night in here, of course (he was half asleep the next morning, after all). He knows Genji will come here first. He knows Genji will see the body, but won’t be able to confirm Kinzo is dead just from the hall. So he’ll have to enter and check. This gives Krauss the opportunity to slip out behind Genji.
From here, it’s pretty simple. He books it upstairs to the VIP room. He locks the window to throw off suspicion. But now he has to make it back to his room, and if Genji heads straight to the west wing, they might run into each other. Thus, the vase. He smashes it, and waits somewhere out of sight near the stairwell. With Genji checking out the east wing on the second floor, Krauss is free to head back downstairs to his room.
And that’s about everything. He had prepared the letter in advance to seal the illusion of the golden witch’s return. Planting it was trivial, as most of you guessed. All he had to do from this point on was ensure that he was “on guard”, to eliminate the possibility of explaining the loop away with a key switch or the like.
Rather humorous that Krauss put together such an elaborate illusion, but when it came to the culprit of the theft, he really left very little room to suspect anyone else (if one didn’t buy the Beatrice solution, of course).