What does the Hardy nature do?

Hardy is a neutral nature. It does not raise or lower any stat.

Can I change a Pokémon to Hardy?

Yes. Use a Hardy Mint to apply the Hardy stat effect to any Pokémon without changing its displayed nature.

Nature

Neutral: no stat is raised or lowered.

No stat is raised or lowered.

Filler nature; functionally identical to the other four neutrals. Fine when you cannot get a perfect nature.

What does the Hardy nature do?: Hardy is a neutral nature. It does not raise or lower any stat.
Can I change a Pokémon to Hardy?: Yes. Use a Hardy Mint to apply the Hardy stat effect to any Pokémon without changing its displayed nature.