I left it up to the imagination of the reader. It could be one of our minds after an imperfect cloning, modified by someone for "efficiency." It could be super-optimizing AIs. Tens of thousands of years from now, thousands of light-years distant, who knows what it will be like?
It seems the consensus in this subthread is that gradual replacement involves a potentially unsolvable philosophical problem, unlike cloning. How can you be sure you remain you during gradual replacement, and don't turn into some kind of a philosophical zombie?