Having played Black for the first time, I'm not sure on this answer: twice I caught Tornadus after it started roaming the region, and both times it had an Impish nature. Is Tornadus's nature set permanently after it starts roaming in the region? Or is this just dumb luck on my end? Please let me know, as Tornadus is planned as my 6th team member.
No It is random. No ingame Pokemon is nature locked .