This Immortal is a science fiction novel by
Roger Zelazny, originally published serially in 1965 in
The Magazine of Fantasy and Science Fiction as "...And Call Me Conrad."
Conrad Nomikos is one of a few million people still living on Earth centuries in our future. Most people have left Earth, many to work for the Vegans. A high-status Vegan is going to visit Earth, and Conrad is recruited to be both tour guide and bodyguard.
It becomes clear as the story progresses that Conrad is not quite human. He calls himself a kallikanzaros and has these identities going way, way back...
One thing that sticks in my memory of this book is the so-called riddle of the kallikanzaros - "Feathers or lead?"
The answer, according to Conrad, is whatever the kallikanzaros wants it to be.