In September of 2013, not long after the reveal of Metal Gear Solid V's scantily-clad sniper, Quiet, director Hideo Kojima took to Twitter to talk about one of his latest creations.
"I know there's people concerning about 'Quiet' but don't worry," he wrote in a series of messages. "I created her character as an antithesis to the women characters appeared in the past fighting game who are excessively exposed. 'Quiet' who doesn't have a word will be teased in the story as well. But once you recognise the secret reason for her exposure, you will feel ashamed of your words & deeds."
For a long time people theorised about this "secret reason for exposure." Now that the game is out, and we've learned the explanation behind her outfit, are we ashamed of our words and deeds? Well...
SPOILER WARNING: DON'T SCROLL PAST THE DOG UNLESS YOU WANT SPOILERS ON QUIET'S CHARACTER
OK. So. The reason Quiet wears that bikini outfit is that she breathes through her skin, using photosynthesis to absorb air, water, and nutrients. She has to do this because her body is infested with parasites.
At the beginning of the game, the villain Skull Face sends his soldier Quiet — in standard military fatigues — to assassinate Snake. She fails. Snake's hospital roommate, Ishmael, manages to set her on fire, and she falls screaming from the hospital window. As we find out later, Skull Face was able to save her life by infecting her body with magical parasites that both give her special abilities and turn her into a bioweapon.
Parasites — the new nanomachines! — are a key part of the plot in Metal Gear Solid V, and I won't go too much into detail here, but to sum it up: Because of these parasites, Quiet can't speak in English, and she can't wear normal clothing. During one scene later in the game, soldiers have forced her to put clothes over her bikini, and you see her come close to suffocating.
It's never quite clear why these parasites affect different characters in different ways, though. Sometimes they kill people right away; other times they heal wounds and give their hosts special abilities. It's also never explained why The End from Metal Gear Solid 3 doesn't have to wear a bikini for his own photosynthesis, which is very similar to Quiet's. (The game even implies that The End had a version of the parasites in question.)
Also, it's worth noting, if you sit with Quiet in your helicopter for a little while, Kojima makes her do things like this:
Which leads, of course, to the question: Are you ashamed of your words and deeds?