This feature might go unnoticed, but apparently, starting from Gen 5 (I think), Trainer cards have a "Trainer's Nature" Section on it. I am currently playing Black 2 and I was making my Trainer Card signature, when I then suddenly got reminded of the Trainer's nature.
I know about that section since the start of playing Black 2 but I never really mind about it. My first Trainer Nature was 'Hardy' but after restarting I got 'Lonely' nature instead (sad lol). I played it various times in different devices and I kept getting 'Hardy' nature, so I assume that you will only get Hardy. This time, however, I got Lonely instead.
This is my Trainer Card: