When are natures set?

[cxoa]

In colosseum, when are the natures set, cos i want a decent raikou, and he fainted once, will his nature always be the same now, or will it change upon each encounter?

 

[ZNtelle]

Natures are set when the battle begins, from my understanding. I remember resetting for and getting an Adamant Tyranitar.

 

[cxoa]

so does that mean each battle, or only the first time?