Basic Trope: A fighter in full armor or some other form of gender-masking attire is revealed to be female.
Straight: Bob is saved by a warrior wearing a full suit of armor. The warrior is revealed to be Alice when she takes her helmet off.
The armored warrior appears to be the pinnacle of masculinity. Underneath the armor is Alice, an incredibly feminine woman.
It isn't revealed that Alice is a woman until the very end of the last episode.
Downplayed: Bob is saved by one of a clan of ninja. He only finds out the identity later, much less the fact that it's Alice, a female.
Justified: Alice's armor suit is either designed for men or designed in a very unisex manner.
A warrior in very sleek, feminine-looking armor takes off their helmet. Inside the suit is Bob.
We're led to believe that The Hero is out to Save the Princess, but when he finally pulls off the rescue, we see him rescue Prince Bob.
Alice, the boyish-looking Action Girl, is revealed to have once been the famous hero Albert, having pulled a Sweet Polly Oliver to join the then all-male hero team, before eventually disappearing for some reason (and, obviously, resurfacing just in time for the team's membership to be opened to females as well).
Exploited: Empress Evulz's attack on what was presumably an all-male group utilizes some substance that targets the Y-chromosome. Alice, inside the power armor and not having a Y-chromosome, pretends to be afflicted so that when Empress Evulz's forces pass her not-dead body, she gets up and pours a ton of rounds into their backs.
Defied: Alice deliberately uses one of the aversions of this trope to make sure that nobody mistakes her for a guy.
Discussed: "Hey, wouldn't it be hilarious if that big armored guy over there turned out to be a girl?"
Conversed: "This is one of those moments where a tough guy swoops in, saves the day...and turns out to be a girl."
Deconstructed: Alice has never before been seen without her armor, thus does all her work with everyone under the assumption she is a man. After The Reveal, the supporting characters take her less seriously, and feel she is "less up for the task" while ignoring her previous achievements to the contrary.
Reconstructed: Alice proves herself as a much more than capable warrior in a Take That to the men who doubted her.
Played For Laughs: Alice's armor even changes her voice... When she's in her armor, she speaks with Basso Profundo.