[Prodigious]

Originally Posted by Village Idiot

Not to mention

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Some characters have completely different personalities depending on what storyline you're playing in.

For example, Fate/UBW Shirou is completely different from his HF self.

I kind of agree but I think that

Spoiler:

It's less that their personalities are different, and more that they are put into different situations. For instance, Shirou is so different in HW because he essentially abandons his dream of being a hero of justice, and wants to save Sakura instead. That alone changes everything because before then he had always been focused on helping everyone. While his core personality was the same(IIRC, he was basically killing himself by going against everything he had believed in), he made a choice which directly affects everything else about him.