Define reusable content --- preferably in the body or else in the header

This is exactly what macros are for. You can define your own macros with \newcommand{\command}{<actions>}, and redefine them with \renewcommand. Both commands can be given anywhere in the document, so you could do something like this:

\documentclass{article}
\begin{document}
\section{Section A}
\newcommand{\PersonName}{Person A}
Some text written by \PersonName.

\section{Section B}
\renewcommand{\PersonName}{Person B}
Some text written by \PersonName.
\end{document}