::you wouldn't know Makoto from Street Fighter 3 was a girl if it wasn't for her dizzy animation.
Well, you could also read her bio... Leo's bio, on the other hand, doesn't say anything about a gender, or about the character trying to look like a boy.
It's pretty easy to come up with some evidence that Makoto is a girl, but I've yet to see somebody pull that off with Leo...
something which keeps coming up is that Leo is a boys name. Well considering the she's obviously trying to look like a boy (probably so Kazuya doesn't recognise her so her revenge will be easier to gain - yeah, yeah, speculation) it makes sense that she's actually called Leona and Leo is (a) a shortened version of that and (b) conveniently plays into the tomboy image.
Actually, I could think of a more plausible explanation for the name: if, like you just theorized, Leo wants to stay incognito, I would think she (assuming Leo's a "she"... still your theory)'d better change her name... Maybe she's not really called "Leo" at all. Maybe the name is part of the "boy" disguise. 'Seems more logical and simple to me, anyway.
Remember, in Japan tomboys will use "boku" rather than "watashi" and will get called "kun" rather than "chan" - all things that technically explicitly mark them as male.
Not sure why you're saying that... Does Leo refer to himself/herself as "boku"? Besides, if Leo is a girl posing as a young man, she could go as far as to call herself "ore," really...
Anyway... Namco kept Leo out of a list of the Tekken 6 women/girls, and the character has been said to be male several times in Japanese magazines. So if Leo is a girl, she's secretly a girl... and there has been no big reveal as of yet. In fact, there's even a possibility Namco is trying to keep Leo's real gender a riddle and has no intentions of confirming it one way or the other, ever. Yumeji (from the Samurai Spirits series) would be a precedent. 81.56.14.31