\llap (or \rlap) at the beginning of an indented paragraph

\llap is not a LaTeX box command and does not start a paragraph.

\documentclass{article}
\begin{document}

\parindent=2cm

\makebox[0pt][r]{x}Hello

H\makebox[0pt][r]{x}ello

\end{document}

You can also use

\indent\llap{x}Hello