When is the nature and gender of your Partner Pikachu/Eevee determined?

Question

.... I don't have to say more, do i?

Answer 1

Nature is determined when you initially catch it, so save before you go to Oak in the tall grass north of town. Gender is determined basically right when you turn the game on. If you want a specific gender you need to delete your save data and try again until you get the gender you want.

Source

Hope I helped!