You know, Back Cover makes everyone believe that there is no traitor and that the MoM set the whole thing up. But we don't see the whole picture. Remember that BC was supposed to show us one side of the story, but people ignore what happened in X and Unchained X.
There are some reasons which lead me to the conclusion that there must be a traitor.
1. The one who "bears the sigil"
It's clear that if there is no traitor, this detail would be unnecessary. I doubt that it's meaningless.
2. Nightmare Chirithy
The sole existence of a dark Chirithy means that it can be traced back to someone whose heart belongs to the darkness. This Nightmare Chirithy seems to follow his own agenda as he collects Lux and don't want the player to know "the truth".
3. Ava vs Luxu
While Ava confronts Luxu, we see Gula talking to the player and Skuld about the lost page. It contains this passage:
"Unable to permit disharmony, you will be disappointed by fate, and lose sight of true strength. Misreading the truth, you will venture forth in secrecy..."
"And then with that one strike, a bell will toll for the final battle and the battle will begin at last, and the time shall be chosen"
It's worth noting that the scene changes back to Luxu and Ava after Gula said that. After that Luxu said the truth about the traitor to Ava and she attacks Luxu with "that one strike". This is heavy information because it gives us a concrete idea who the traitor could be.
So there must be a traitor after all. Who is it? According to the given information the traitor must
- bear the sigil (it does not necessarily mean the "X", it could be some other form of sigil, for example the Dream Eater symbol)
- disappointed by fate (this implies that the traitor had good intentions first, but was disappointed that everything will lead to a war)
- misreading the truth (this means that the one must have read the Book of Prophecies and made up his own interpretation)
- venture forth in secrecy (this means that the traitor has been plotting behind the scenes ever since he read the book)
This means that only the following persons can be the traitor:
Why Ephemera? He is the one who wants to solve the "mysteries of the world". In the result he is interested in the book of prophecies and could have seen important passages of it at some point in the story. After this he was disappointed of the listed events and disappeared for quite some time. He is driven by curiosity which eventually leads to corruption by the darkness (the same pattern as Xehanort and Riku).
Why Gula? We know that Gula and Ava are good friends, maybe even more. Her reaction during the Luxu fight could indicate that the traitor is someone close to her. Gula is the only one who knows what is written in the Lost Page, so he could "misread the truth" about the traitor. He is also the first who collects Lux in order to summon Kingdom Hearts. The whole time he is someone who is secretly doing his own thing.
Why Ava? According to the book of prophecies the strike on Luxu started the conflict, which makes Ava theoretically the cause of the Keyblade War. Even if she doesn't fulfill the criteria for the traitor yet, there is a clear impression that her actions make things worse instead of better. Her conversation with Luxu makes her a suspicious candidate, although she has good intentions.
Why Luxu? He is the only one who didn't get a book of prophecies. So what if he took a look at one of these? Maybe he started to realize what the MoM's true intentions are. He is also the most obvious when it comes to the "sigil".
By the way:
The X is often referred to as the sigil. In DDD Young Xehanort said that Sora has the "X" on his chest and they always knew where he is due to the X. During the Xemnas fight he said: "As your flesh bears the sigil, your name shall be known as that of a recusant". It's just a short sentence but it has very high significance now.
A recusant is someone who refuses to obey people who have authority. In the context of Back Cover, the one who bears the sigil (the traitor) is a recusant. This means the traitor refuses to accept the MoM's teachings or his vision of the future and tries to change it. In fact, it's not about light vs darkness anymore, it's more about destiny and changing events that are predetermined to happen.
Xehanort has a deterministic nature and believes everything is destined to happen, although he is curios if destiny can be changed. The answer for this lies in the Keyblade War, where "darkness prevail and light expire". In the end he wouldn't be disappointed if darkness won't prevail, because it's only about satisfying his curiosity. Sora would be the exact opposite, following his heart without any restrictions or boundaries. He doesn't let his life be determined by destiny, he forges his own destiny. This makes him somehow the biggest traitor of them all and he will actually succeed in changing the future. After all, Sora and Xehanort both come from Destinys Island.