From the moment Leo was announced, in the earliest of Tekken 6's trailers, fans across the world have been debating Leo's gender. On sight it really is hard to tell, if not possible at all, no matter how many times pro male/female debtors try to convince us. This is why I am eliminating this factor all together, as one person's "male-Leo" is another person's "female-leo" in their eyes.

The way I'm going to put forward my theory, is going to be solely on what Namco have given us, so I'm discounting any theories put by fans which include; Leo attempting to hide her/his true gender by dressing up like the opposite sex. The only thing we know official, story wise, is that Leo had a parent killed by Kazuya and that Leo is out to get revenge on him. Having put that out the way, I am now going to try to convince you , why I believe that Leo is male.
I have personally come to the conclusion by these following "clues", that I found out in the research....

1. Heihachi Mishima to Leo: "You lack proper manners boy "
In Scenario Campaign Heihachi Mishima refers to Leo; as a "boy" , if you do not believe me then feel free to either click on the image below , to enlarge, or pick heihachi yourself and fight the Leo boss in Scenario Campaign.

2. Anna Williams to Leo: "I'd love to help a cute boy like you"
This is the second time; we have another Tekken character refers to Leo as a boy. Again if you do not believe me, then feel free to click on the picture below to enlarge, or play as leo in scenario campaign and fight Anna Williams, as boss.

3. Voice Actor: Is female BUT voices Ash Ketchum from Pokemon !

Ok so the argument could be that Leo's voice actor being female (Veronica Taylor), proves that Leo is female too...right ? Maybe, but if you look at the character of Leo, you will notice that Namco, keep reminding us how young this guy really is. We have Heihachi Mishima referring to him as a "child" we have Anna Williams calling him "cute", namco certainly do be little Leo every so often.
Now if you look at the majority of the young males/boys in anime and cartoon, you will find out that they are indeed voiced by females and not males. A classic example would be Bart Simpson from the Simpsons. The voice actor for Leo has already voiced a young male before, in the popular 90's cartoon; pokemon, and so Namco may have found her as the perfect person to voice another young male; Leo.

4. Tekken Trend Games

Males out number Females in the Newcomers department
(A side from Tekken Tag, Tekken 2,3,4,5 have had more male new comers than female.)

Tekken 2:
Females = 2  Angle, Jun Kazama
Males = 6 Jack 2, Lei, Bruce, Baek, Roger, Alex

Tekken 3:
Females = 3 Ling Xiaoyu, Panda, Julia
Males = 7 Jin, Eddy, Hwoarang, Forrest Law, Bryan Fury , Ogre, True Ogre, I have excluded King II, Tiger, Kuma II for arguments sake

Tekken 4:
Female: = 1 Christie
Male: = 2 Steve Fox, Marduk

Tekken 5
Female = 1 Asuka
Male = 2 Feng Wei, raven have again excluded Jinpachi

So you see namco have always had the trend of more males than female new comers. If we count up the female and male playable new comers, excluding Leo, we would have 2 female Alisa and Zafina and 3 male Lars, Miguel and Bob. However if you added Leo to the female list, then female new comers would be equal to male. But if I were a betting man I would certainly see the odds of having 4:2 males as a more realistic ratio, due to the gender historic trends.

5. Secret Male/Female Poses: Leo Doesn't slap but headbutts, while Wang doesn't think much!

If Wang fights a female, occasionally he would have a special intro pose saying: "you're hot..."
This has so far been addressed to every singly female in the game, and it has never been reported that it was addressed to Leo before.
Secondly everyone by now must be aware of the secret slap retaliation females would respond to after being slapped by Anna Williams. If you pick Anna Williams , and if you slap a female opponent ( back + 2, 2) as long as the other female is cpu controlled and the fight does not take place in practice you will find any female fighter respond in Anna Williams slap in the exact same way. This again excludes Leo had she been a female.

EDIT

I've just now been informed of a male only secret taunt, which Leo also does. If you select Heihachi and you headbutt a male ( forward 1+2 ) the male opponenet would reply with the same headbutt. (Go to 2:30) in the video below.

Leo does the male only taunt, and ignores the female only taunt.

6. Minor Observation:

- Leo was in the male category in the official art book

- Leo was referred to as "his" on namco's official website, but then again the very website claimed Asuka and Jin were brother and sister XD

- No sauciness , Tekken as a series has been getting saucier and saucier...not quite DOA material but getting there.

- No female tantrum when Leo loses, I have seen every female so far, except zafina I confess, do the unique female tantrum after she loses the first round. Leo does not do it either.

Conclusion

I know I'm going to get a lot of hate for this, but I'm sorry I'm just going by what Namco has given us. Apart from the voice actor argument, every bit of proof I put forward, was namco's official work or word. The only argument left for Leo being female, is that Harada allegedly referred to him as a she in 2007. Other than that, until Tekken 7 comes out, Leo is a male in my eyes. From the 1st of February Tekken Nation will put out a poll on whether you think Leo is male or female. Make sure that you take part in it :)

Deebo