gistfile1.tex

\documentclass{minimal}
\usepackage{minted}
\usepackage[LGR]{fontenc}
\usepackage{fontspec}


\begin{document}
  
Problem: β is not recognized. UTF8 characters have a red box surrounding them.
This is not really ruby, but hey.

\mint{ruby}|infl[β2] := {}|
   
 \begin{minted}[showspaces]{python} 
 def boringØ (args = None):
  pass 
 \end{minted}   
 
\catcode`β=\active
\defβ{{\fontencoding{LGR}\selectfont b}}

And here the β character gets folded out.
 \mint{ruby}|infl[β2] := {}|

\end{document}