At this trip I just came back from, I decided to hunt down Latios. I finally cornered him on Route 24 with a lower-leveled Pidgeotto, so I chucked an Ultra Ball for lulz (first time seeing him in this playthrough). This is what happened:
Me: Man, I love the Latis--
*ball still shakes*
Oh, it's still going?
*clicks, captured on the first try*
So it's a Naughty natured. Not bad, I guess. I just didn't restart because I think the roamer's natures might be predetermined by game. I think.
I have claimed Pokemon Emerald.