Where the Letter F Breaks the Pattern
Most mathematical styles run A to Z in one unbroken block, but not all of them for this letter. Script capital F is โฑ (U+2131, Script Capital F). It was added to Unicode in the Letterlike Symbols block years before the mathematical alphabets, and the later block left a gap there instead of duplicating it. On some devices a different font draws this character, so a word set in that style can show one letter that looks slightly unlike its neighbours.
Small Capital, Turned and Other One-Off Forms
Small capital F is ๊ฐ (U+A730). Like most small capitals in Unicode it was encoded for phonetic notation, not as a font feature, which is why small caps text pastes as ordinary characters. Unicode has no dedicated turned f, so upside-down text substitutes a character that already has the right shape when flipped. Superscript f is แถ (U+1DA0). There is no subscript f, so subscript text leaves that letter at normal size. The regional indicator ๐ซ is half of a flag: two of them in a row turn into a flag emoji when they spell a country code.
Which F Is Safe to Paste
Bold ๐ , italic, sans-serif and monospace forms display on nearly every phone and computer. Circled โป and fullwidth ๏ผฆ are also widely covered. The squared and dark squared forms of F have no emoji version, so they stay black and white. Screen readers may announce a styled F by its full Unicode name or skip it, so keep anything important in plain letters as well.