You mean another, more compact version of `j` with a shorter descender? Well that’s called an alternate glyph.
Kerning is strictly about the relative spacing between two adjacent glyphs. The only case that would ever be vertical is if you’re writing vertical lines (such as in Chinese or Japanese).
Interesting...
I'm in Japan and I was about to reply that Chinese and Japanese are almost always fixed-width, but luckily I grabbed a product next to me (laundry detergent) covered in Japanese. I was expecting to see perfect line-up of characters on adjacent lines proving they are fixed-width. They aren't even close, and this is true for both vertical and horizontal text on everything I look at. I opened a few apps like word and confirmed by default everything is fixed-width. So TIL Japanese is fixed width for plebs but any professional copy has way more kerning that I realised.
Kerning is strictly about the relative spacing between two adjacent glyphs. The only case that would ever be vertical is if you’re writing vertical lines (such as in Chinese or Japanese).