A METAFONT character

You're running Metafont in proof mode; Metafont loves proof mode and will do anything it can to stay in proof mode unless you specifically command it to produce real font files. Notice that your error message says that it couldn't find a .tfm file?

Try running it this way:

mf "\mode=localfont; input aaa.mf"
gftopk aaa.600gf aaa.pk

This should crank out a .tfm and appropriate .pk file for your font, which should then work in your document.

Put the a.mf where tex can find it (e.g. in the current directory) and then use

\documentclass{article}

\begin{document}
\font\test=a 
\test b
\end{document}