I've always personally thought Leo is a girl. But Namco pretty much left it as a mystery. Some websites say Leo is a girl others say Leo's a guy. I think Leo is a girl. I mean you can prove some things that people say he's a guy.
1. Leo is short for Leonardo. Not true, it can albo be short for Leona.
2. Sounds like a dude. Not really, Leo is voiced by Veronica Taylor, and I've heard girls with low voices.
3. Uses punches. Julia also uses punches.
4. Looks like a dude. Eh, haircut can be for either, I think Leo has a girly face, especially if you compare Leo to the other male characters.
5. Clothing wise, Leo's mother passed away, so Leo was raised by his/her father, Leo could as well be a tomboy. (I mean, i dressed as a guy when I was younger.)
So, there are some reasons Leo can be a girl. But we're just going to have to wait and see.
I say HE is a guy. I jusr got the game 2 days ago and he sounds like a guy, looks like one. He may have a baby face (that looks like a guy) and there may not be ANY facts about him being a man or woman but Im gonna say hes a dude. If im wrong, then im wrong...
Leo's gender has been kept ambiguous by Namco as they "want a character players will love regardless of gender." To further confuse players, Leo has female exclusive hair cusomizations, indicating that she is female, while being placed in the male category of the Tekken 6 Art Book, indicating that he is male.