This contains spoilers for Surge Concerto (Ciel nosurge and/or Ar nosurge) from the games to side materials. Last warning.
When you load a game or start a new one, what is supposed to be happening? Does it reset events on their end, or are you going into an alternate world? Are there countless versions of Ion and the others who, at every step where you reloaded, were abandoned? If you beat the game and reload or start a new one, does that undo it? From the wiki and some events in Ar nosurge it sounds like it is canonical that Earthes can reload or start a new game. If I understand right we are all supposed to have our own Ions and Deltas, and there are many versions of each of us dealing with many versions of each of our versions of them.
The other player seems to operate by different rules. Not only can he communicate freely, but cannot reload. Near the end he has Prim say "I've spent a lot of time leveling her so it would suck to lose her, but I can always make another one." This implies that he cannot reload if Prim dies, but maybe I am misinterpreting this?
Earthes acts like a villain to Delta partway through Ar nosurge, but then says "I'm not like you" to the other player at the end. Maybe the other player is stuck selecting what he says the same way, or selected whatever he thought was the funniest between his choices to see how we'd react. Nothing in English seems to touch on what was up with him. It could be an alternate you annoyed that he got nothing but villainous choices in that game, or an alternate Yuuki surprised at all the coincidences.
Edit to avoid double posting.
I think maybe I get it on the reloading. In order for Earthes to go down a different possibility, he'd have to go into a different world (fifth axis). As for the doomed Ra Cielas, other versions of the same Earthes would go down those paths. The mere connection of Interdimend would insure that every possibility that can happen involving him, does happen. Merely by arranging it so the connections would be made, their world would be saved. As well as not saved, but it is no longer doomed in every outcome.
In this view, the entire universe is completely static, change is only perception moving along an axis. Other than perception, nothing changes. Events can never change, you can only move to a different possibility world even with time travel.
There is a weird, other possibility left. What if instead of going back in time, you rewrote current time to be different? Chapter 7 of Ciel nosurge seems like this. This would mean that instead of going to another world, the future of the one you were on would not follow logically in the way we normally think of it. The future could be set to anything, however seemingly unrelated to the events that proceeded it. Perhaps Interdimend could do this, but it sounds like it canonically does not, at least in Earthes' case. It would be a lot harder than going to another possibility world since one involves changing your viewpoint from a position where time is meaningless, while the other involves large changes to the other world.
Now that still leaves the other player a total mystery. "I've spent a lot of time leveling her so it would suck to lose her, but I can always make another one." is bugging me. Developer oversight? Translator error? Different rules? I'm still wrong? It really is an alternate, gamer Yuuki picking the funniest options because she is bored?
Edited to explain the quote from the other player better.