Natures
Serious does not change any stat. A Pokémon with this nature has exactly the values that follow from its base stats, IVs and EVs.
A neutral nature. It shows up on event legendaries whose creators did not want to impose a role.
Flavor
None — a neutral nature has no liked or disliked flavor. That is convenient with Poffins and Pokéblocks: such a Pokémon eats anything without complaint, so healing never works worse.