[DNSB]

Looking at the OP's message, he references a Kobo Libre. The Libre has a couple of CJK fonts available but you will need to select one of them which will set also set that font as the default for the next book. You also need to avoid using font-family references in the CSS which can block the ability to select either Tsukushi Mincho or UD Kakugo as the display font when displaying Japanese characters when creating an epub that will be displayed with the RMSDK renderer. Creating a kepub will use Kobo's epub3 renderer which will search for a glyph that is not defined in the current font. At time the results of doing this can be a bit strange.