>>14669939Brave is fine for going through the story, but with a female charmander you can just breed it to get what you want later.
If you're not okay with its stats or nature now though and are deadset on getting Charmander, make sure you have a full party when battling Sycamore and when he gives you Charmander, say No. Leave and dump a poke in your box, go back to him and save before you accept it. From there, you can soft-reset easily to get what you want.
