Originally Posted by DNSB
Not sure if this is your issue but I tried the ModifiedLarger variant and it did work for me.
Basically, I had to rename the four files to include the spaces in the font family name and came up with the following (I used Fontforge to see the font family name.)
Charis SIL ModifiedLarger-Regular.ttf
Charis SIL ModifiedLarger-Bold.ttf
Charis SIL ModifiedLarger-BoldItalic.ttf
Charis SIL ModifiedLarger-Italic.ttf
After copying the renamed files to the Kobo fonts directory, disconnecting from the computer and power cycling the Kobo, the ModifiedLarger font worked.
Edit: I tried switching to several English locales, French Canada, Spanish and Japanese and I saw no issues with displaying text style/weight other than oblique not working which it never does.
I've attached the renamed fonts. Please let me know if they are not correct. Thanks.