XY are so easy that Natures matter even less than they usually do. This means any Nature will be fine for your Charizard; don't worry about hunting for one.
That said, for a mixed moveset, a neutral Nature (Hardy, Docile, Bashful, Quirky, Serious) won't hamper any of your stats. You can also go for a Nature that lowers any of your other stats at the cost of your Charizard performing negligibly worse in that department. In other words, it really doesn't matter.