Here is a version adapted from egreg's solution at How to split text into characters:

##Code:

\documentclass{article}
\usepackage{xparse} 

\ExplSyntaxOn
\NewDocumentCommand{\stringprocess}{ m m }
 {
  \egreg_string_process:nn { #1 } { #2 }
 }
\cs_new_protected:Npn \egreg_string_process:nn #1 #2
 {
  \tl_map_inline:nn { #2 } { #1 { ##1 } }
 }
\ExplSyntaxOff

\newcommand{\rotatechar}[1]{\rotatebox{90}{#1}}

\begin{document}
\stringprocess{\rotatechar}{Roated}
\end{document}

Here is a version adapted from egreg's solution at How to split text into characters:

Code:

\documentclass{article}
\usepackage{xparse} 

\ExplSyntaxOn
\NewDocumentCommand{\stringprocess}{ m m }
 {
  \egreg_string_process:nn { #1 } { #2 }
 }
\cs_new_protected:Npn \egreg_string_process:nn #1 #2
 {
  \tl_map_inline:nn { #2 } { #1 { ##1 } }
 }
\ExplSyntaxOff

\newcommand{\rotatechar}[1]{\rotatebox{90}{#1}}

\begin{document}
\stringprocess{\rotatechar}{Roated}
\end{document}

Here is a version adapted from egreg's solution at How to split text into characters:

##Code:

\documentclass{article}
\usepackage{xparse} 

\ExplSyntaxOn
\NewDocumentCommand{\stringprocess}{ m m }
 {
  \egreg_string_process:nn { #1 } { #2 }
 }
\cs_new_protected:Npn \egreg_string_process:nn #1 #2
 {
  \tl_map_inline:nn { #2 } { #1 { ##1 } }
 }
\ExplSyntaxOff

\newcommand{\rotatechar}[1]{\rotatebox{90}{#1}}

\begin{document}
\stringprocess{\rotatechar}{Roated}
\end{document}

Here is a version adapted from egreg's solution at How to split text into characters:

##Code:

\documentclass{article}
\usepackage{xparse} 

\ExplSyntaxOn
\NewDocumentCommand{\stringprocess}{ m m }
 {
  \egreg_string_process:nn { #1 } { #2 }
 }
\cs_new_protected:Npn \egreg_string_process:nn #1 #2
 {
  \tl_map_inline:nn { #2 } { #1 { ##1 } }
 }
\ExplSyntaxOff

\newcommand{\rotatechar}[1]{\rotatebox{90}{#1}}

\begin{document}
\stringprocess{\rotatechar}{Roated}
\end{document}