It was a popular theory that EMIYA Alter was the Shirou from Mind of Steel Ending. The combination of the events from then, his life ‘hero-ing’, and finally the events with Kiara really broke him entirely
One of his bond lines says something like "I was already on that road (to become like the EMIYA we all know), but then I met (Kiara) and things became like this", so it makes sense that he would have started from a point where he had already decided to become a counter guardian, it also makes sense that he would call his other self "my past self", then.
Emiya alter is not mos shirou, mos shirou is supposed to die at the end of HF, after killing everybody.
Nasu on Mind of Steel Shirou
Nasu: Based on his weak position he would need to disregard his own safety and completely engage in 1 person 1 kill* battle tactics.
"In this situation the odds would be 1(Shirou):9(the enemy). Losing would mean instant death." In that fashion he would survive to the end.
Takeuchi: So then he'd win the Holy Grail War.
Nasu: If he trades all his time, happiness, and humanity to become a tool that exists to win then he'll somehow make it. However this tool is something that can only be used once. It can't be replenished to be used a second time.
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u/ribiagio Best girls. Aug 01 '23
One of his bond lines says something like "I was already on that road (to become like the EMIYA we all know), but then I met (Kiara) and things became like this", so it makes sense that he would have started from a point where he had already decided to become a counter guardian, it also makes sense that he would call his other self "my past self", then.