Disclaimer: I own nothing of the Buffyverse.
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He knows. He knows that one day he won't be able to get up and put his persona of Spike, the Big Bad. That one day they will see him for what he truly is.
William.
Angel calls him that still. Clinging onto the past, of what his Childe used to be. But even he doesn't notice that it is William. Sure, Spike was there for awhile, but then it was back to William. Wearing Spike's skin.
One day, he won't be able to do it. Make snarky comments, smoke, annoy everyone. Be Spike.
William fears that day as it is getting closer every day.