Why is the symbol for "cathode" a 'K'?
I would think that it's because of the origin of the word: kathodos 


  Greek \$\kappa\alpha\theta\omicron\delta\omicron\varsigma{}\hspace{7pt}\$ (kathodos), descent : kat-, kata-, cata- + hodos, way, path.


That and to avoid confusion with C which is already taken by capacitors.