documentclass{article}

%usepackage[charter]{mathdesign}

DeclareSymbolFont{greeksymbols}{OML}{ppl}{m}{it}
DeclareMathSymbol{pii}{mathalpha}{greeksymbols}{25}
DeclareMathSymbol{chii}{mathalpha}{greeksymbols}{31}
DeclareMathSymbol{mui}{mathalpha}{greeksymbols}{22}

begin{document}
{Huge $pii chii mui $}
{Huge $pi chi mu $}
end{document}