The best nature for Charmander is the Modest and Timid nature. Charmander’s best stats are Special Attack and Speed, so natures that amplify these two stats are best to make the most of your Charmander’s innate skill.
Choosing Your Starter
Charmander is one of the three options available to you as a Pokémon starter when starting out the game.
Charmander cannot be found in the wild, and the only way to get one is through choosing it as your starter Pokémon at the beginning of the game, or trading for it with a friend. This means you’re stuck with the nature of the Charmander you get at the start of the game.
As mentioned, Charmander’s best stats are Special Attack and Speed. Special Attack involves all elemental and type damage. In Charmander and all its forms’ cases, these would be all its Fire-type moves such as Ember, Flamethrower, etc.
Charmander’s Speed also means it would almost always be the first hitter in every battle, unless it goes up against a foe faster than itself.
It is up to you to decide which stat to prioritize, taking into account the randomized nature that your Charmander may be assigned.
A Modest nature means the Special Attack stat is increased by 10%, but the Attack stat grows more slower than the others. This is good if you want a Charmander/Charmeleon/Charizard that will be a hard-hitter. A well-landed Flamethrower can instantly KO any opponent.
A Timid nature means your Charmander/Charmeleon/Charizard will be one of the fastest Pokémon in your party. Timid natures up the Speed stat while it decreases the Attack stat. A lower Attack stat isn’t too big of a deal if your Pokémon is quick – first hits per turn may mean the enemy won’t even be able to touch you.
Choosing Charmander’s Nature
Natures are randomly assigned to your Pokémon when you get them. The only way to get your preferred nature for your Charmander is to spam Save and Load until you get the Charmander with the nature you want.
Here is an article explaining how to get the desired nature for your starter
While Modest and Timid are the best recommendations for your Charmander, you can still work with other natures as it’s a matter of training and leveling up your Pokémon to the best it can be. Why not take your Charmander as is and do some EV training for its Special Atack and Speed stat instead?