[gogu1904]

Something is not right. Or, I do not understand (which sounds more plausible). I start with the downloaded Bitter HT font and work from there, in three manners.

(a) Modify with ReadifyFonts the fonts Medium and MediumItalic, to rename them to Regular and strip panose info. Add the downloaded fonts Bold and BoldItalic.

(b) The same four files as above. Modify ALL of them via ReadifyFonts.

(c) The same four files as above. Edit the two Medium fonts via FontForge. Add the downloaded fonts Bold and BoldItalic.

I tried with Kobo KEPUB reader (with store KEPUB books) and with KOREADER with sideloaded EPUB files). The only option that works is (c). Otherwise the fonts are defect (letters like B, d, p have their "eye" filled in black).

This is not consistent. For some other fonts, the three options are equivalent.