Thanks Jon for the new version of Charis. I have been testing it (once I finally managed to persuade Mediafire to let me download the file - any reason for not attaching it to your post?).
The following also applies to your Charis SIL font.
First I renamed the files to use the Regular, Bold, Italic, and BoldItalic suffixes that the Kobo seems to use.
The main problem with both Modified and Modified Larger is getting Kobo firmware post 2.1.5 to display italics, and I have had very inconsistent results. At first Charis Modified seemed to be OK if I used the default US locale, but not if I changed the locale to UK (https://www.mobileread.com/forums/sho...d.php?t=206200
). Since I have added on Modified Larger I can't get either font to display italics, regardless of the locale (I am cold rebooting after changing locale). That is, I can't get italics to display with your Charis fonts even on the default US locale.
I have my own version of Charis which does work with US and UK locales, on firmware 2.4.0. For this I renamed the font to Charisas (removing all spaces in the name, and internally changing the name), and added a United Kingdom language to the font. I do not have time to definitely test all the different combinations to determine exactly which change I made that has got the firmware to use the italics, but I do know it works where other fonts don't.
I have also raised this problem with Kobo support; they requested a sample epub that displays this problem, which I sent to them, and it has gone to second tier support. Fingers crossed this will be fixed in the next firmware release.