So just as thing there are, unless the writers forgot, 3 Daybreak towns.
The original the war happened in, a second the Dandelions escaped to in order to avoid a war, and a third digital one they use to access a datascape in order to collect lux. Now until we know better that means that there are in reality 21 vessels to be used HOWEVER we take away 7 of those now because 7 are to be used to escape the digital world so in reality that means there are only 14 that matter. So now let's look a this objectively...
Of the 7 digital vessels 1 is taken by Maleficent leaving 6 and we have exactly 6 people we know need to escape this digital world. Ephemer, Brain, Lauriam, Ventus, Skuld, and Elrena with the latter I would assume be coming along because of Lauriam.
Of the 7 Second Daybreak town vessels 1 is taken by Maleficent to return to her own time and 1 is taken the "true" dandelions. This means only 5 vessels remain and this lines up with Luxu's secret report in KH3 saying he saw 5 people escape this world. Ventus, Skuld, Lauriam, and Elrena definitely travel to the future as we know and/or have heard about them being there. This leaves us with two people who are more iffy which are Ephemer and Brain.
It's clear one of these two is going to be the one who sets up scala and the regeneration of the worlds. All signs of this point to only one person and that is Brain. Scala's keyblades are modeled off starlight and master's defender of which the latter is both blade passed down through the order AND was originally Brain's keyblade. Second while we don't know exactly they set up the worlds to rebuild themselves they must have some sort of model to draw on, something with the data they need, aka the book of prophecy which is also in brain's possession. There is other evidence that whoever set up Scala has the book namely it's prophecies are known things, things taught to the students of scala, which is why Xehanort and Eraqus were both aware of them. Lastly Eraqus is supposedly descended from one of the dandelion leaders it can't be Ephemer and Ventus as they are still kids long after he is a dead old man, Marluxia and Larxene are young adults after he is a dead old man, and Skuld is 99% subject X meaning she arrived in the future after Eraqus is already an old man. Mix this with Brain having Master's Defender and looking extremely similar to Eraqus and it equals out to yes Brain did take one of the vessels. I also want to note Eraqus calls him his grandfather (not great grandfather) and the world is only partially reconstructed, so chances are that Brain jumped hundreds of years into the future and then started this whole world rebuilding process only maybe a hundred or so years ago from Dark Road Times.
This means Ephemer stays behind and that makes a lot of sense. Ephemer is the overall leader of the Dandelions and basically the Player's best friend so it would be bad form to just ditch them. In fact KH3 shows that Ephemer must still be somewhere with the Dandelions given that he sent their power to aid us in KH3. So yeah Ephemer is most likely the one who did not travel BUT he might have found a way to rescue the Dandelions from the virtual prison, but who really knows.
What that means though is that the usual suspects for the true dandelion are able to be pretty easily accounted for and reach their destinations without Luxu. Ergo I think that can only mean that it's none of them and thus I believe there are only a few possible options....
1. It's a new character we haven't seen before
2. It's a character we have seen before but is an outlier like maybe Yozora or Strelitzia
3. It's Demyx
I'm going with 3 because it would plug up some holes. Marluxia, Larxene, Demyx, and Luxord all are connected to the secrets of the ancient past of the keyblade but as it stands Demyx is the odd man out. Marluxia and Larxene are known past wielders who we've seen the general origins of now. Luxord has some kind of relationship to Verum Rex given that he's most certainly Yozora's driver, the better ending lets us see the top of head in the mirror, it sounds like him, and Luxord's VA confirmed he was voicing the driver so yeah. But what about Demyx?
Why isn't there a shred of a hint to Demyx's presence or past? Why is he even here in the organization so constantly when barely contributes anything of value their plans besides just being more meat? How can someone so cowardly and weak have such a powerful heart to have a full human Nobody? Xemnas said the only reason he is here is because he has secrets inside him. Demxy implies the only reason he ever came back to the Organization is because they basically strong armed him. Basically there isn't much of a case for Demyx to even be in this story and his list of accomplishments are nearly entirely non-existent. Despite being on the bad guys side he offered them no meaningful help and when he came back to their side a second time he actively worked against team darkness from the inside. Traveling around gathering up vessels and taking them to people who could fill them up with guardians of light, being in part directly responsible for Roxas's return and subsequently saved multiple guardians of light by doing so. And how did he do this? By being such an unassuming off the radar unknown factor that he could slip around and do stuff undetected.
This is why I believed Demyx is the true dandelion because when you look at the story Demyx was this lone little person planted into organization, a seed of light in the darkness, who due to his nature is cast to and fro wherever the winds take him just like an actual dandelion seed. His nature ensured he would never do anything big that would upset the greater plans at work, but his unassuming presence would allow him to do small things that had a large ripple effect. He would be just the kind of thing Luxu needs, a way to plant light into darkness that doesn't endanger the MoM's plan but does mess up the forces of Darkness. Given Luxu would have chosen him practically on a whim and sent him forward it would almost mean that Demyx is a Dandelion with no meaningful backstory and no grand transportation, Luxu just probably knocked him out and transported him. It's spur of the moment and unceremonious which fits the complete blank that is Demyx's story.
Anyway that accounts for the 7 virtual vessels and the 7 secondary daybreak town vessels but we still have the original 7. Given the MoM's words we can safely assume he used one of them meaning there are 6 left and we know Luxu didn't use one of them as the MoM's plan hinges on Luxu just living for literal eternity via body snatching until some vague appointed time in the future. So 6 vessels need to be taken and I can believe this works in one of two ways. Either they were all destroyed when the world was swallowed by darkness which is perfectly feasible I think to believe. OR 5 of these 6 vessels were taken by the Foretellers explaining why they just disappeared from history without a trace and have shown up in KH3, with Ava deliberately changing her destination to head somewhere else. Of course that means they would need to know about the ark to do so BUT if Luxu told Ava everything then Ava would know and that would explain why Ava went to the keyblade war when she was against the fighting, because she went to inform the others and save them. That leaves a solitary vessel behind and I could believe that was just destroyed.
