phasma strikes me as having an archaic, important sounding name, like victoria or adelaide or helena. something very feminine and traditional (maybe it was her mother's name, or her mother's before her? where pasma's from men might take their wives names so phasma's full name would be something like Victoria Charlotte Bruenlanthe IV. she goes by phasma because old-world titles like that are looked down upon by the first order, and besides, the first three victorias in her family were noted political leaders who supported democracy and the republic and that is most certainly a Bad Thing here)

hux, meanwhile, never tells anyone his first name because it's something dumb-sounding. important generals name their kids like rockstars do, so hux's first name is something like Meteor or Exhaust-Port or Macncheese.