The runes are chosen to optimize early damage at early levels to help give you a leg up on the opponent and play heavy harassment.

I feel the masteries are self explanatory because you'll most likely be doing mid, so the defense is helpful, and you'll be relying on the runes for your AP mostly, but if you want to go full on glass cannon and build offensive mastery, that's your choice too.

This build is to optimize damage early on, and then to help with mana regeneration and cooldowns. Then Increase mana, movement, and damage, then increase defense for late game skirmishes.

With Ahri's Orb of Deception and the amount of damage you'll be putting out, you'll be able to farm up a row of minions in one or two shots, and clean up the rest with foxfire. This will also help you get extra gold for the more expensive items you'll need later on.

For the love of Inari (the Japanese goddess of food whom is portrayed as a fox) she is a support assassin. Don't go running in thinking you can take on the world 'cause you're very squishy, and you have high cooldowns until you get the Evil Tome. just stick by teammates and serve as the damage output, and let them finish off. So you don't get a kill. Assists are just as good. Having high assist count is better than having a high death count.