The backwards K is normally used to denote a called third strike in a strikeout. It’s typically written on a scorecard. I’ve been looking for the backwards K so I can denote the strikeout looking on Twitter, and I finally found it:
(for unsupported browsers — Chrome)
The easiest way to use this character is to copy and paste the backwards K from above and save it in a note or something you can copy and paste from routinely. This character is actually from Apple’s implementation of the Unicode from the artificial, Latinized version of the Lisu alphabet. This alphabet contains an upside-down, turned K which looks similar enough to a backwards K I think this pass on Twitter.
If you don’t see the backwards K in the block above, you computer or mobile device probably isn’t using a font that supports that specific character. It’s supported on Macs and iPhones (as well as the Edge browser in Windows 10).