Here are the assumptions upon which all my reasoning is based.
- First, the overall objective of the fake is to delay and weaken the Braves as much as possible. There are two possible strategies s/he can take: one, delay the group whilst removing as many of them as possible by making sure that s/he is the last person any of the others would suspect. Or alternatively, to make it such that the barrier can be deactivated upon the death of the first victim, so that the party will continue with the fake still in their midst, ready to sabotage them further into the Demon God’s territory.
- Second, the Saint of Salt and the Saint of Seals, who were both instrumental in creating the temple and the barrier system, were both honestly working against the revival of the Demon God. Hence, the sword and altar that we saw are necessary for activating the barrier, and the seals do indeed protect against allowing fiends entry. Similarly, the Saint of Fog that created the barrier that bars entry or exit from that mouth area really intended it as an aid to the Braves.
- And finally, the mystery is also good enough to impress some serious mystery fans, so it cuts out anything that relies on strong coincidences, or information that isn’t at all hinted at in the show/novel.
At this point, the only characters I would say are 100% clear are Adlet and Flamie. If Mora's statement about the petals disappearing upon the death of a true brave are true, then those who have desisted from killing Adlet though they had the chance could simply be bluffing in accordance with the first strategy. With regards to the two I've named, however, unless Adlet is deceiving Hans and Mora about the ‘fiend-detecting’ solution, Flamie has to be clear. And there is no good reason that Adlet would lie about that if he was the fake, unless the two of them were working together. However, their numerous interactions belie that. The only alternative would be if he was the fake but really did fall in love with her…but that would be one of the unbelievably stupid developments that I just can’t see mystery fans accepting.
So, we are down to five suspects, which the requirements for activating the barrier should help us narrow down. The discovery of Riura’s body backs up Adlet’s hypothesis that the fake (and the fiends s/he is working with) deceived the others into believing that the barrier had been activated before it actually was. The sword being in the altar is not necessarily a smoking gun (only the Saint of Swords would probably be able to manipulate it from a distance, but who’s to say it wasn’t in the altar in the first place?).
However, we know that the barrier had already been activated by the start of episode 5, where Hans and Adlet find that they cannot leave the area. The only people we see touching the sword (and possibly the altar/slate) during that time were Adlet, Goldov and Nachetanya: Goldov took it out of the altar and Adlet tried to deactivate it using the method he’d heard about, before Nachetanya took it from him. We see her swinging it around wildly, breaking the slate and then yelling out “I will be the mistress of the barrier.” She presumably puts it back into the altar before Chamot starts speaking, for she no longer has it in hand at that point.
Hence, if the sword and the altar are required for activating the barrier, only one of these three could have done it. This also means that Lauren and the remaining soldiers at the fort were indeed cooperating with the fiends, and lying about the method of activation, but that’s not really important for determining who the fake is.
Now the precise process of activation becomes important. Given that we don’t know what the actual activation method is, any of the three can still have done it. Even Adlet is not excused, because he may have unknowingly activated it when trying to do the opposite. In fact, this would play perfectively into the fake’s hands, because Adlet’s death would indeed release the barrier. The major problem I have with this idea is that it requires the propagation of a lie in the legends about the previous generation of Braves, from 300 years before.
Now, I highly doubt that the Saint of Salt would make the activation something that could be done by mistake, but I won’t rule it completely out. If this was the case, then the mystery is still wide open, as any of the three could have unknowingly activated it. If so, however, then the fake--more likely to be Mora than Hans or Chamot--is basically winging it, because this accords them very little control over how to ensure s/he is one of the last to be suspected. Hence, I think this is rather unlikely, especially since it doesn't strike me as something that mystery fans would be particularly pleased by.
Which leaves us with this: if the activation method is more difficult to guess, then the best option for the fake is to be the one who activates it. This affords him/her the best opportunity to reduce the strength of the Braves before they even get close to the Demon God. Having ruled Adlet out, we’ve left with Goldov and Nachetanya…and what the latter did with the sword makes her the most suspicious on this point. I can't actually rule out Goldov, but there are a few other pieces of circumstantial evidence that make me more suspicious of the princess. First, Nachetanya was the person who sent Adlet ahead to the temple — if she was indeed the fake, this would ensure that she knows exactly who would come under the most suspicion, given the characteristics of the temple’s seal. Furthermore, the fact that she’d traveled so far with him lends support to her stringent initial defense of him, thus reducing the chance that she’d be the next key suspect. If she is the fake, framing Adlet is actually the most effective option for her. Nachetanya's also told Adlet a couple of times that he's a little too trusting of people - her being the fake puts a real twist on those words of hers. Then, IIRC, there were at least two characters who were a bit like "oh, I didn't think you'd be chosen as a Brave." Though given how strong Nachetanya is shown to be, that's not a deciding factor for me.
And that’s basically why Nachetanya’s my key suspect. Though I'd be up for Goldov being suspicious of her but covering it up because of his loyalty to her - I'd just be pretty disappointed because it's not as interesting a 'twist'. If there's any doubt in my mind, it's that she genuinely looked angry and hurt as she was about to kill Adlet, and truly shocked to see Hans. But if she's been acting the entire time, it's still completely believable. The only other alternative is really Mora (especially given how Chamot and Hans apparently have some info./news to share), based on her behaviour - but in her case, she'd be an unwilling fake who's being blackmailed into what she's doing. I'd find that to be a bit of a cop-out, though, given the lack of hints as to a motive or means (for framing Adlet).
Alright, all set for the finale next week!