Can you chose your pokemon's nature?

[Satycorn]

Hi everyone,
I'm relatively new to this forum and new to playing pokemon. I have played Leaf Green only and now I'm about to get Pokemon Pearl version. I didn't know what pokemon natures were, they'd appear on LG but I knew nothing about them. I learned more today when I decided to do more research.
So basically, the pokemon's nature is a pokemon's personality and it is indicated by the flavor it likes, the stat it accelerates the growth of and the stat it accelerates more slowly.
But I wanted to know if can you chose or "pick" your pokemon's nature, or is it set by random?
So let's say that I really like Lucario and I want to get one. It is available in Pearl by getting Riolu first. And from the nature's list, Adamant seems to be the best nature, since it boosts the attack and the Sp Attack raises slower, and Riolu/Lucario is a Fighting pokemon, thus not needed the Special Attack.
Is there any way I can pick or "change" my pokemon's nature before/after getting it, or do I have to restart the game every time until I recieve a Riolu with this nature?
Thanks!

 

[Oryx]

There's no way to affect a Pokemon that you're catching in the wild. If you breed one, you can use an Everstone held by one of the parents to affect the nature. You can look all this up on Bulbapedia if you have any further questions, or ask in the thread for the game you're playing in the proper section.