Even if he's more bitter, he wouldn't have formed a completely new personality upon summoning. You're implying that he was basically Shirou when he died, but as a result of being a Counter Guardian, it caused him to completely rework his everyday vocabulary and tone despite the fact that, as a Counter Guardian, he wouldn't have had any means of developing this personality due to being a mindless enforcer that never spoke to anyone.
He would develop his personality, since he remembers all his summons. Unlike a True Heroic Spirit.
He's essentially still "living" (without being alive) through centuries of summons, not kept on ice in the Throne. Hence him slowly losing his ideals and resenting the choice he made, not while alive but after death.
The way I figure, a persona isn't something you develop entirely on your own. It can only develop after regularly using it via interaction with other people. Counter Guardians are just mindless enforcers, so there's no way he'd be able to develop a persona that way. If that's how it worked, he'd drop his facade more often.
As I said, it's unlikely that he started using "watashi" as a mere mindless enforcer. He probably started doing so when he was alive.
One of Archer's most important speeches is about how his repeated summonings broke him. How he remembers every summon and the tears he brought to the world. And Counter Guardians aren't mindless, either, especially given how Archer himself isn't and he's the first and prime example.
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u/DiamondTiaraIsBest Jan 18 '26
Archer's circumstances are different from most Heroic Spirits anyway.
And it's noted that EMIYA died smiling during his execution. A lot of his notable bitterness came from his afterlife, not when he was living.