Japanese pitch accent numbers
Retina 0.8.2, 68 bytes (UTF-8)

\d+(.[ャュョ]?)
$1/$&amp;$*1\
/1\b

/.
/
+`1(1*.)(.[ャュョ]?)
$2$1


Try it online! Link includes test cases. Explanation:

\d+(.[ャュョ]?)
$1/$&amp;$*1\


Convert the number to unary, wrap it in /\, and insert it after the first mora.

/1\b


If the number was 1, then delete it and the /, leaving the \.

/.
/


Subtract 1 from the number, but in the case of 0, this deletes the \, leaving the /.

+`1(1*.)(.[ャュョ]?)
$2$1


Move the \ one mora to the right for each remaining 1.