Where the Letter C Breaks the Pattern
Most mathematical styles run A to Z in one unbroken block, but not all of them for this letter. Fraktur capital C is โญ (U+212D, Black-Letter Capital C). Double-struck capital C is โ (U+2102, Double-Struck Capital C). These were added to Unicode in the Letterlike Symbols block years before the mathematical alphabets, and the later block left a gap there instead of duplicating them. On some devices a different font draws these characters, 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 C is แด (U+1D04). 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 c, so upside-down text substitutes a character that already has the right shape when flipped. Superscript c is แถ (U+1D9C). There is no subscript c, so subscript text leaves that letter at normal size. The regional indicator ๐จ is half of a flag: two of them in a row turn into a flag emoji when they spell a country code.
Which C Is Safe to Paste
Bold ๐, italic, sans-serif and monospace forms display on nearly every phone and computer. Circled โธ and fullwidth ๏ผฃ are also widely covered. The squared and dark squared forms of C have no emoji version, so they stay black and white. Screen readers may announce a styled C by its full Unicode name or skip it, so keep anything important in plain letters as well.