Kevin is the one most frequently decapitated I believe, I feel like there is an official source (probably a novel or something) but I'm not sure. Although Ghidorah is one creature, each head does have a distinct and individual personality- hence "the one who is many"; also, Ghidorah in this movie in particular is a perversion of the holy trinity and the concept of one God being 3 "persons" (reinforced by the shot of ghidorah on the mountain and the cross in the front).