I have to add that in the litter earlier this year of my pedigreed Siamese (old style , aka Thai ) queen Phai-lin, the most hissy, screamy little kitten when she was under a month old, turned out to be maybe the sweetest, most lap-loving kitten by 12 weeks old who adored cuddling with everyone. This hissy little girl turned out to be SUPER cuddly. So I agree it is definitely not a sign they will turn out " mean" !