\documentclass{ltjarticle}
\typeout{\meaning\kanjiseriesdefault}
\makeatletter
%\DeclareFontSeriesDefault[sf]{md}{b}
\def\mdseries@mc{el}
\def\bfseries@mc{sb}
\def\mdseries@gt{l}
\def\bfseries@gt{k}
\DeclareFontShape{JY3}{gt}{k}{n}{<-> HaranoAjiGothic-Heavy.otf:jfm=ujis}{}
\DeclareFontShape{JT3}{gt}{k}{n}{<-> HaranoAjiGothic-Heavy.otf:jfm=ujisv}{}
\DeclareFontShape{JY3}{gt}{l}{n}{<-> HaranoAjiGothic-ExtraLight.otf:jfm=ujis}{}
\DeclareFontShape{JT3}{gt}{l}{n}{<-> HaranoAjiGothic-ExtraLight.otf:jfm=ujisv}{}
\DeclareFontShape{JY3}{mc}{el}{n}{<-> HaranoAjiMincho-ExtraLight.otf:jfm=ujis}{}
\DeclareFontShape{JT3}{mc}{el}{n}{<-> HaranoAjiMincho-ExtraLight.otf:jfm=ujisv}{}
\DeclareFontShape{JY3}{mc}{sb}{n}{<-> HaranoAjiMincho-Bold.otf:jfm=ujis}{}
\DeclareFontShape{JT3}{mc}{sb}{n}{<-> HaranoAjiMincho-Bold.otf:jfm=ujisv}{}
\def\TEST{%
 \par
  A: \f@encoding/\f@family/\f@series/\f@shape.
  K: \k@encoding/\k@family/\k@series/\k@shape.
  あいうABC
}
\def\TESTA{%
  \TEST NORMAL
  \emph{\TEST} EMPH
  \textsf{\TEST} SF
  \textbf{\TEST} BF
  \textgt{\TEST} GT
  \textbf{\textgt{\TEST}} BF+GT
  \textbf{\textgt{\textmd{\TEST}}} BF+GT+MD
  \par 
  \textsc{\textit{\TEST}} SC+IT
  \par
  {\fontseries{b}\selectfont \TEST} B
  {\fontseries{bx}\selectfont \TEST} BX
}
\typeout{\meaning\kanjiseriesdefault}
\begin{document}
\makeatletter
{\ttfamily
  \meaning\kanjiseriesdefault, \meaning\kanjiseriesdefault@kernel
}
%{\ttfamily\expandafter\meaning\csname bfdefault\endcsname::%
%  \expandafter\meaning\csname bfdefault@previous\endcsname}\par


%\def\bfdefault{b}\newpage
{\ttfamily ======DEFAULT \string\bfdefault: \meaning\bfdefault ======\par}
\TESTA

\def\bfdefault{b}\newpage
{\ttfamily ======\detokenize{\def\bfdefault{b}}======\par}
\TESTA

\def\bfdefault{bx}\newpage
{\ttfamily ======\detokenize{\def\bfdefault{bx}}======\par}
\TESTA

%\ttfamily
%\expandafter\meaning\csname bfseries \endcsname
\end{document}
