Question

How much do Natures and Characteristics affect a Pokemons stats?

Answer 1

Natures:

>A Pokémon's Nature usually affects the growth rate of two of its stats, ultimately increasing one of its stats by 10% and decreasing another by 10%.

Source

Characteristics:

Characteristics do not affect a Pokémon's Stats it just tell players which Stat contains a Pokémon's highest IV.

>A Pokémon's Characteristic indicates which stat contains a Pokémon's highest IV.

Source

Answer 2

Natures increase a stat and decrease a stat by 10%.
Characteristics show which stat the Pokemon has the best IVs in, at level 100 the Pokemon gets 1 stat point for 1 IV, the max amount of IVs being 31.

Here is a page on characteristics.

Hope I helped!

Source: Experience

Answer 3

Natures decrease and increase a certain stat by 10%.

>A Pokémon's Nature usually affects the growth rate of two of its stats, ultimately increasing one of its stats by 10% and decreasing another by 10%. Natures also determine the Pokémon's favorite flavor and its disliked flavor.

Which stat it increases and decreases depends on the nature.

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>A Pokémon's Characteristic is determined by the remainder of its highest IV divided by 5.

Characteristics determine what number of IVs a Pokemon can have in a certain stat.
For example: a Pokemon that "loves to eat" can have 0, 5, 10, 15, 20, 25 or 30 IVs in HP

IVs are one of the most important things in determining stats. From Generation 3 onward it uses this formula to calculate a Pokemon's HP stat and other stats.

Hope I helped!