Thief of Time - Immortal

And as far as Susan was concerned...

Well, she was partly immortal, and that was all there was to it. She could see things that were really there, she could put time on and off like an overcoat. Rules that applied to everyone else, like gravity, applied to her only when she let them. And, however hard you tried, this sort of thing did tend to get in the way of relationships. It was hard to deal with people when a tiny part of you saw them as a temporary collection of atoms that would not be around in another few decades.

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The Thief of Time - p85