To be honest, I don't find Sinon THAT great so far. I like her, but I don't like how she treated Kirito after she found out he was a male. Yeah, he saw her avatar undress, but he showed her his ID so he didn't look like a creeper. She was the one who pretty much made the assumption that Kirito was a girl and he just went along with it to get help. I don't think I ever heard him verbally lie saying that he was a girl.