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PROS :
Extreme poking
Good CC
Extreme Burst
Long Range
Free Flashes
Short Cooldowns
CONS :
Very low base HP
Very weak to CC


Counters
I've played Ahri against almost every mid i can think of, and lost only to a few.
Ahri is countered mainly by stuns and long range. Here's a list from the most dangerous at the top.

1. Xerath
Oh my god ridiculous range. he will never subject to your orb of deception tease because he's sitting halfway to his base and still hitting you.

1.5. Mordekaiser
not really a mid, but i've played against him a few times and boy let me tell you, his moves hit like a damn truck.

2. Anivia
Her long-range slow/stun ability completely tears Ahri up early game if you can't dodge it.

3. Annie
Stuns everywhere, once you're stunned you're helpless to her full burst combo.

4. Lux
Shoots the orb thing halfway across the lane.

She can pretty much take care of any champion in lane that doesn't have very long range or readily availble stuns.

Anything with ability power is A-OK

Basic caster masteries, just get everything a caster needs in offense then mana in utility.

What i normally go with is starting with a Sapphire Crystal, a Mana Potion, and a Health Potion. Why so much mana? Because the way I play kicks *** because It takes so long for me to run out of sustain and in that while I can push very hard and often get first blood against a careless lane opponent. After I get a first kill (or if i don't, doesn't matter) or die to a gank or a counter (counters explained later on), or run out of mana, mana potions, and clarity, B back and buy your Boots of Speed, then Ruby Crystal. Eventually build up to a Rod of Ages. according to the enemy team composition, you can either build a Deathfire Grasp or a Morello's Tome. both serve the same purpose. After you have these two, build up to a Rylai's Crystal Scepter, and if the game is going this long, get a Rabadon's Deathcap. You can buy whatever after you have the rabadons.

Unlike most guides and ahri's i've seen, i get Foxfire first. Why? Because unlike her other abilities, you can skill up foxfire and it's mana cost won't go up. If you're against slightly low-ranged champions like Twisted Fate, you can surely land almost every hit on him before he can get out of the way. Always get Orb of Deception first though, it's a very good poke-tease to do some decent damage. After first orb, get Charm for it's good utility, then stack fox fire and orb, then charm last, getting ult 6/11/16. The good thing about Foxfire is that you can use it while running. I've been saved so many times at level 9 with 5 points in foxfire when i'm running away at low health. Just pop your fox-fire and suddenly they're at the same health as you, then you can charm and orb and they begin to run again. Pop ult and ignite and you got yourself a kill.

Thanks for reading! I hope this guide helps you to get some 23/4 games in your match history somewhere.