ERAM is a ghost possessing the MANTLE game.

ERAM Might be a Ghost Lightner possessing a video game.

The Sword Route, on a surface level, most closely resembles the horror trope of a "haunted video game".

A mysterious evil entity possesses a game (A la Red from Godzilla NES), it starts showing a bunch of creepy imagery (like killing your family and friends, like the angel in Godzilla NES), and the video game can oftentimes physically harm you, like in the Godzilla NES creepypasta, Godzilla NES was an extremely influential creepypasta for Toby, as Flowey was based around the NPC "Face", so not only do we know he's familiar with Creepypastas, which was honestly pretty obvious, but we know that one of them influenced him to a pretty large degree.

I believe that ERAM is a simple ghost possessing a video game, and unlike many explanations for what the Sword Route game is, I feel like this explanation is prettt easy to wrap your head around and doesn't introduce too many brand new concepts for what types of entities exist in this world.

Most people don't really mention this, but, ERAM, the boss of the video game, resembles a generic bed-sheet ghost, it uses this specific "cross-bomb" attack that is most prominently associated with Mettaton, Mew Mew, and Mettaton NEO, all bosses that are either ghosts or are heavily connected to a ghost.

When the most likely Shadow Mantle flies away after King's defeat, it resembles a ghost possessing something and flying away, rather than someone using magic or telekinesis to take it.

A ghost would have no trouble possessing Ramb and kidnapping him and that's a shocking enough sight for the Ribbick to respond in that way.

A ghost can phase through wall, disappear, and give Kris the OddController without Ramb seeing it.

If it's a Lightner, then it can go anywhere in Hometown, unlike most Darkners.

Speaking of the NEO body, assuming Mettaton is ERAM and ERAM is the "Strange Someone" that I think Pink's new dialogue implies, that would kinda simplify the whole NEO body thing, Mettaton takes his lost dreams and uses them as a weapon to entice Spamton into the Weird Route. That would also mean MTT gave Spamton the Thorn Ring.

Spamton is also an obvious parallel to Mettaton in general, with the name, NEO body, and (former) celebrity status.

For some strange reason, Burning Eyes uses the "Power of NEO" or "Undyne the Undying" leitmotif. I also noticed that instead of being associated with "True Heroes" like MTT Neo and The Undying, who were giving up their lives to save rhe world, it's instead associated with "True Villains", Spamton, who orchestrated the Weird Route, and ERAM, who heavily encourages it.

I believe Pink's body (notably not the ghost, who would have known Mettaton) met Mettaton, who lived right next door to Asgore, is the cousin of her future possessor, in UT he was Alphys' best friend and Alphys is a whole other can of worms with how suspicious she is. It could have easily been Mettaton who introduced the angry ghost to the doll. Since it's a doll, it has to be SOME sort of person, I think?

Also, there's the fact that we give the major character Tenna over to MTT, and I find that interesting because I feel it means they'll both probably be more relevant later on, otherwise why make the quest if both are just gonna stay in the background and disappear forever?  And the character arc is about Nobody coming out of his shell, so he should come outside.

ERAM, Mettaton, and the "Thank You" ARG responder all have a similar tone. They show sadism, sassiness, and a bit of flamboyance, tones Mettaton constanyly uses.

"Don't make such a sour face". (From the ERAM fight, not the ARG)

"Aww."

"Oh, you found that, too! Perfect, just perfect... Thanks for all your hard work, Kris!"

*"There! That's what I wanted to see! Flickering red, like pretty little flames... Your eyes can't hide it, Kris. Without play... The knife grows dull".*

The Thank You Responder makes multiple gameshow references in this "sassy" manner.

"Ding! ding! ding!"

"What is this, a game show?"

There's a hint of theatrics to the whole thing here.

When the ARG goes into those wild fantasies before saying "...maybe not.", it reminds me of how Mettaton went into a speech about Alphys trying to become the hero before ending it with "...or not".

It's honestly pretty unlike most of the mystery characters people suspect to be important, especially Gaster, obviously the "not too smart" Friend, and even Seam. Seam also has the issue that they're just a doll and can't really be in Chapter 3 and there's no implication Seam could control a video game like this. If it's a ghost, it simplifies the whole thing and makes it so Seam's arc and dialogue aren't just bold faced lies, which COULD be the case, but generally it's more clever to have a lying character be partially honest and lie by omission and half truths.

So why Mettaton out of the three ghosts? Why can't the Mantle Holder be its own character?

I don't think ERAM makes sense as its own character, if you're good enough at the Knight you never have to see ERAM and even then ERAM in the entire game has like thirty seconds of screentime. I think the fact that it's skippable and has such little dialogue would make more sense if ERAM was simply a disguise rather than its own thing.

Aside from the personalities:

Mettaton in Undertale has a history of becoming a violent dictator if given too much power in his Neutral Ending. He isn't a great person until the Genocide Route.

On a meta level, Mettaton is the first clear example in UTDR of a "vessel" and "spirit" duo, he has a magenta SOUL, his appearance is quite similar to the Flowers in Chapter 5, being this "almost-humanoid", he's the second humanoid character we see and the first one aside from Frisk...

But aside from that, MTT in this game is hopeless and has given up on his dreams, and as such may be more susceptible to becoming "secret boss-pilled" like Spamton, Jevil, and WR-Noelle. This weird ERAM alter-ego may be a shy character using escapism to try to exert power over the world.

I also find it odd that we give Mettaton, such niche character, Tenna, such an important character and I feel like it would make sense if it these guys showed up in person to complete their character arc.

How does Mettaton know all this crap about Kris?

Well... Maybe MTT can possess the' vessel, which may be a clone (or original version) of Kris?? The vessel clearly looks like Kris. This would also explain the "second Kris" the Ribbick sees which is seemingly not the pixel guy. I don't think it's something like the Headband, as popular as it is, as I think the vessel is just the only "clone" aside from Hero\_Sword with any foreshadowing. (Even if this isn't Mettaton or a ghost, I think the clone is the vessel).

I just don't see the point of making a fourth clone of Kris, it not being HERO\_SWORD is already a rugpull, but instead of it being the only other "Kris clone" it's a brand new Kris clone with no build up? Don't vibe with it.

Perhaps possessing the vessel fucked up MTT's world view if they were flooded with the emotions of Kris, an obviously mentally ill kid. When Pink was possessed her body's emotions and "Doki's" managed to affect the ghost's emotions as well, and these emotions were sometimes uncontrollable.

If the vessel isn't playable and is relevant it would also need a pilot, a SOUL, to control it, right? It seems as if, strangely, the SOUL isn't very different from a ghost, it may be possible for a ghost to possess something as supernatural as the vessel. (The vessel will obviously be relevant due to the Thrash Machine being a much smaller example and getting both a boss fight and a playable appearance)

I also feel like the line "No one can choose who they are in this world" coming from the depressed "Nobody" who proceeds to steal our vessel would go REALLY well with this theory.