This might be a little odd to say, but Billie has a particularly pretty neck. He doesn't have a large Adam's apple, which I think makes him more attractive. Unlike other men, it's more smooth and elegant. I think that concavity at the base of his neck between his collarbones (technical term is suprasternal notch - 'supra' meaning above and 'sternal' referring to the sternum) is more obvious on him than other guys and I think that makes him even sexier. I guess the point is that his neck is yet another aspect that makes him an incredibly beautiful person.