$$
\newcommand{\HIRANO}{\unicode[sans-serif,STIXGeneral]{x306E}} % HIRAGANA LETTER NO (U+306E)
\begin{array}{cc}
x_k - \mu & \text{$x_k$の偏差} \\
x_k - \mu & \text{$x_k\HIRANO$偏差} \\
\end{array}
$$
\[ \newcommand{\HIRANO}{\unicode[sans-serif,STIXGeneral]{x306E}} % HIRAGANA LETTER NO (U+306E) \begin{array}{ll} x_k - \mu & \text{$x_k$の偏差} \\ x_k - \mu & \text{$x_k\HIRANO$偏差} \\ \end{array} \]
https://twitter.com/watiko/status/624858733974171648
http://docs.mathjax.org/en/v2.3-latest/tex.html#unicode-support