Well you gotta switch your spartan to female, then beat the campaign as fast as possible. You gotta get between 3-10 hours to lose the helmet and if you manage in under 3 hours she wears a black bikini. Remember to save the little emu and koala aliens as you run to your ship at the end.
Seriously though the pathfinder sorta looks like Samus' visor. It's the only one that might make me reconsider my EVA. --- "Sir, I don't think I'd care to look at a nun's legs. It'd be very impolite." "You'll just have to force yourself Godfrey. This is war!"