Dont take the haircut seriously! : (
The mainpoint is that Xion would be girlboy, not physically, but she tries to act like boy maybe get some rispect or something else. You gotta admit that her hair is extreme short in front at least.
Because Roxas and Namine are kinda from Kairi and Sora, and Nomu knows what else.. A nobody has the memories of its original, therefore Namine having Kairi´s memories and Roxas would have Sora´s, but Sora turned back so Roxas had no memory of Sora.
But Xion might appear as girl like Kairi, but has Sora´s memory since Roxas didnt get it. This would make Xion want to be Sora´s nobody.