well on smogon it says latios and latias natures are set when steven shows you their picture BUT if i KO them then beat the elite 4 will latios's nature change? right now it has impish, kind of the worst nature possible to get. This question has probably been asked already but none of the answers I saw were of any help.
I think the answer is NO because I did I test I took my three soul silvers and all 3 ended as IMPISH nature when I beat the elite 4 1 time 2 times and 3 times hope I helped :D