As others have said, we see the sigil
applied throughout the series in 2 ways: the mark on Sora’s chest in DDD (supposedly) and as a magical X (chi) mark which is inserted (presumably as some sort of curse/spell) into one’s name. The only time we actually see someone get branded by the sigil is when Roxas wakes up in Twilight Town, but ReMind appears to confirm that this process we see Xemnas use (creating an anagram of one’s name and inserting an X) is, indeed, the very process of marking someone with the Recuscant’s Sigil.
A couple important questions come up:
- We never see Sora get marked in this fashion, nor does he have an X in his name. Question #1: Are there ways of marking someone with the sigil other than incorporating it into their name? I, personally, don't think the mark that YX talks about on Sora's chest is actually the mark on his clothing. YX's quote is "That sigil on your chest is the proof." Since we see the sigil literally emerge from RAX's chests in ReMind, I can almost guarantee this scene in DDD will be retconned to mean Sora has been similarly marked, but we're not seeing it.
- My personal theory is that Sora shares the mark with Roxas, which explains how Sora can be marked without having an X in his name. Yet to be seen. Which raises Question #2: can the sigil be carried across a person's "other selves" (be they a person's Nobody, a Nobody's "somebody", or another incarnation)?
(Side note: worth also keeping in mind what MX says about the letter X (chi), as he holds in his palm what appears to be the Sigil itself. "Death... a letter that spells endings." I think it would not at all be far-fetched to think of the Sigil as a kind of curse, which may very well span lifetimes and time itself.)
With all this in mind, something I’ve been thinking about lately is the Lost Page and the mention of the traitor as “the one who bears the sigil.” I know many speculate the traitor to be Ava - and it’s perfectly plausible that she was, at some point, branded with the sigil - but given the information we
do have, it struck me that this statement could have a new meaning after the revelations about the sigil in ReMind.
Given the information we have at hand (though knowing full-well there is important information we're missing), I think Back Cover + ReMind may imply that
Luxu is the traitor. The reasoning is simple: the only instances we've actually
seen someone's sigil mark are Roxas, Axel, and Xion, and based on the scene we see of Roxas being marked, we can assume the same holds for Axel and Xion as well. Additionally, given that in ReMind, Axel is now Lea, we now know for certain that a person can remain marked with the sigil even if they were marked as a Nobody and have since been re-completed (this partially answers
Question #2 above in the affirmative). With the Axel-Lea example I think it would be perfectly reasonable to presume Sora inherited Roxas' Recuscant's Sigil when Roxas re-joined with Sora in KH2. If this is indeed the case, we have no reason to assume there is any other way to mark someone with the sigil than what we see ("cursing" someone's name), which gives us a tentative answer to
Question #1: "probably not/there is no reason to believe so."
Given everything we know about the sigil it is almost certain that Xigbar (aka Luxu) bears the sigil, having been marked the same way as Roxas/Axel/Xion as a member of Org XIII. Additionally:
- Luxu, in his original incarnation, also has an X in his name. This could very well be nothing, but it does raise the question - has Luxu always been marked? (The fact that Back Cover hints at "the one who bears the sigil" and includes one foreteller with an X in their name strikes me as pretty on the nose, but that's just me. Could very well be a red herring).
- Assuming Xigbar definitely bears the sigil, does this retroactively apply to his past selves/lives? Raises Question #3, to which I have no answer: Does the sigil transcend time?
- Or, if Luxu is already marked (given the X in his name - but if so, by whom? MoM?), then the tentative answer to Question #2 would suggest that Xigbar has already inherited the sigil, though it would then be unclear if Xemnas needed to mark him again for his own purposes.
In either case, whether Xigbar was himself marked, or the "X" in Luxu's name is significant, I think there is a strong case to be made for Luxu/Xigbar being the traitor based on this information.
Other questions obviously remain, such as what the true purpose of the sigil is (is it just tracking? or is there more?), and whether it's tied to the one who marks the cursed individual (for example, would Xemnas have to re-apply the sigil in order to track someone who's already been marked by someone else (say, the MoM)? EDIT: I’m thinking the answer to this latter question is yes, given what Roxas says about how their sigils will disappear along with Xemnas.
Lots remains shrouded in mystery. But it's pretty clear this thing will have a large role to play. Also, anyone please feel free to poke holes in my argument.