documentclass{article}

newcounter{person}
newcommand{newperson}[1]{%
  refstepcounter{person}%
  #1~(Ptheperson)}

begin{document}

Today I have met David~(P1) in the parking. David was friendly.
David is the father of John~(P2). Suddenly I remember he was a friend of mine, Susan~(P3).

Today I have met newperson{David} in the parking. David was friendly.
David is the father of newperson{John}. Suddenly I remember he was a friend of mine, newperson{Susan}.

end{document}