I can't. Once I go back and do everything that needs to be done, I will disappear. The fake will switch places with the original like nothing ever happened and I will go back into the nothingness where I belong.

[ Because that's what he is, after all-- he's nothing. Not in the proverbial sense, but actually quiet literally. Once it's over, he can no longer keep this form of Emiya Shirou.

Everything will shed away.

His memories. His physical being. Even his personality. Maybe having his abilities sealed is what makes him continue to be a person to begin with. ]

Even if someone were to take me with them, they would have to form a contract with me and provide me with magical energy. But as for my identity-- you seem rather well read. Have you ever heard of the name Angra Mainyu before?