I was trying to get Apple's new
New York font to work on the Kobo for fun (because why not?) since it's a reading font now used in iBooks, but no matter what I did, it wouldn't render correctly. The internal and external font and family names were correct, the weights were consistent throughout, I tried it with and without PANOSE data but it would always render as italicized.
Long story short, it turns out that "Style Map" in the OS/2 section also needs to be set correctly as well, either to Regular, Italic, Bold, or Bold Italic depending on the font. Apple had set it to Regular across all four font types, and as soon as I edited that section, everything worked.
So just a heads up in case anyone encounters the same issue when working with any other fonts after following all the previous suggestions on how to make this stuff work: Make sure the Style Map section is set correctly too!