Is K.C. an acronym for any of the old pso characters from the first Phantasy star games?
I can answer that...

No, as far as I can tell, K.C. is not an acronymn or a reference to any previous Phantasy Star characters. The only ones who come close are;

Kainz Ji An (Josh Kain - PSII), Kain Sa Riik (Rhys - PSIII), Louis Kay Escher (Alair - PSIII), Luna Kay Escher (Kara - PSIII), Shess Tierney (Kyra Tierney - PSIV)

In both the US and JP translations - there are no real matchings of names for "K.C." or "Kasey". Nor would he have any personality, apperance, or background connections with any of the above characters.

I didn't check Gaiden or Adventure thurougly, but I doubt you'd find any connection there either.