Well obviously she's a girl because it is ZELDA in disguise!
No offense, because I'm in no way attacking your personal character, but this sort of response has driven me up the wall for the past (almost)12 years.
There are two very separate parts to the argument: the first being the physical gender of Sheik, and the second being the mental gender and portrayal of Sheik.
[] P H Y S I C A L []
This is the first thing that most people argue about when it comes to Sheik. Some people believe Zelda simply dressed as a male while some believe Zelda took on a male transformation. I'm going to address both:
&& DRESSED AS A MAN
It's a very common thing in both real life and fiction that women would dress as men and vice versa. James Gray, Marinus, and Billy Tipton are only a few of these women.
Cross-dressing is also pretty popular in Japanese/Korean(and now more often Chinese) pop culture, seen a lot in manga and even in music. Characters and people in real life who did they usually are small breasted and/or breast-bind as well as "pack". Zelda is a fairytale, so it's not surprising they used this method of hiding an identity.
However, when women did this, they were not dressing in form-fitting clothing as Sheik did. They were dressing in overalls, pants, baggy shirts, robes, etc. etc. They dressed in clothing that could easily conceal any feminine bumps and curves.
However, there are bindings in plain sight around Sheik's upper body.
It's also said that Sheik's eyes were originally going to stay blue, but I've never seen any interview or quote about this, so I have no idea if it's true or not. It's possible that Zelda simply muffled her voice and stuffed all her hair up into that turbin thing. And, then there's the question- if Princess Zelda could change into a man,
why bother hiding her face? Who'd recognize her?
&& TRANSFORMED INTO A MAN
This is also another popular idea when it comes to the Sheik gender debate. While simply dressing in male's clothing is a popular way of disguising one's self, actually transforming into the opposite gender is more popular when it comes to fiction (manga, anime, magic, etc.). Pardon me for getting a little graphic, but I think this is okay to talk about: breasts can be bound, yes. Some women are flat-chested. Princess Zelda has always been an A cup (in US sizes) save for the more recent TP in which she was more like a small B Cup. She'd have no problem binding her breasts. However, the major flaw with this arguement is that Sheik is not shaped like the common, slim "shounen" character- he is trim, but he is
muscular. No matter how skillfully a person binds their breasts, they will not achieve the look of masculine pectoral muscles.
Sheik has pecs. Even women who do great workout routines will not get this look. It is something that Princess Zelda
could not have achieved with a female body and breast-binding.
Not only does Sheik have pecs, but his entire physique is entirely masculine in shape. By the time Sheik is helping Link out, Zelda is Link's age.
Why is it that Princess Zelda has curves and Sheik does not? The official art for both characters are drawn at the same angle, and the body types are completely different from each other. Sheik is trim and muscular. Yes, women gain muscle too, but not the same way men do. A muscular man will look very different from a muscular woman. Sheik's eye color is also different, as is his skin color and shade of blonde hair color. His entire appearance is different from that of Princess Zelda's. The masculine musclulature comes and goes in an instant. Sheik "transforms" in a flash of light rather than pulling down his mask.
I'd also bring up the manga. No, not every manga is canon. However, it's been stated by Aonuma himself that the manga is a way to make changes to the story. Akira Himekawa drew it out. So guess who thought up the little stories? Yes, it's Aonuma's own idea that Princess Zelda
"would become a boy." Impa seals Zelda's consciousness away. But even then, it doesn't delve into the physical. Perhaps Sheik, in the manga, is female and just doesn't know it. Strangers things happen in manga/anime, but I doubt it in this case.
[] I D E N T I T Y []
Regardless of Sheik's physical gender being male or female, the fact stand that the disguise Princess Zelda took is one of a man. He fools Link, Ruto, everybody. Regardless of what is beneath the clothing,
the identity of Sheik has a male gender. This was Princess Zelda's intention, and so the character she "became", whether it was by magic or simply crossdressing, is a man.
That means that even though Sheik is Princess Zelda who is a woman,
Sheik the character is still male, because she chose to become a man. But Sheik, while a male character, is a a woman in the end.
Really, Sheik isn't Princess Zelda. Princess Zelda
chooses to be Sheik, even if only for a couple years. There's something to think about
CONCLUSION: That's up to you. My own
personal conclusion is that, physically, Sheik is a man regardless of being Princess Zelda. Many will disagree, and that's fine. This is a debate that will never come to an end, so I only hope that my wall of text helped anybody who was willing to read through it in its entirety.