Ahri is a very squishy um.... .foxy?.. champion who can disrupt teamfights.

Unlike most AP Mage Carries, Ahri has very high mobility at level 6 and can gank very easily while the enemies are not alert.

LORE
Unlike other foxes that roamed the woods of southern Ionia, Ahri had always felt a strange connection to the magical world around her; a connection that was somehow incomplete. Deep inside, she felt the skin she had been born into was an ill fit for her and dreamt of one day becoming human. Her goal seemed forever out of reach, until she happened upon the wake of a human battle. It was a grisly scene, the land obscured by the forms of wounded and dying soldiers. She felt drawn to one: a robed man encircled by a waning field of magic whose life was quickly slipping away. She approached him and something deep inside of her triggered, reaching out to the man in a way she couldn't understand. His life essence poured into her, carried on invisible strands of magic. The sensation was intoxicating and overwhelming. As her reverie faded, she was delighted to discover that she had changed. Her sleek white fur had receded and her body was long and lithe, the shape of the humans who lay scattered about her.

However, though she appeared human, she knew that in truth the transformation was incomplete. A cunning creature, she adapted herself to the customs of human society and used her profound gift of beauty to attract unsuspecting men. She could consume their life essences when they were under the spell of her seductive charms. Feeding on their desires brought her closer to her dream, but as she took more lives, a strange sense of regret began to well within her. She had reservations about actions which never troubled her as a fox. She realized that she could not overcome the pangs of her evolving morality. In search of a solution, Ahri found the Institute of War, home of the most gifted mages on Runeterra. They offered her a chance to attain her humanity without further harm through service in the League of Legends.

"Mercy is a human luxury... and responsibility."
―Ahri

Lets take a look at Ahri's stats, Ahri:"Shall we?"
Health 380 (+80) Attack damage 50 (+3)
Health regen. 5.5 (+0.6) Attack speed 0.668 (+2%)
Mana 230 (+50) Armor 11 (+3.5)
Mana regen. 6.25 (+0.6) Magic res. 30 (+0)
Range 550 Mov. speed 330

Ahri's passive is Essence Theft

Ahri gains a charge of Essence Theft with each enemy unit hit by any of her spells, with a cap of 3 charges gained per spell cast. Upon reaching 9 charges, Ahri's next spell will have 35% bonus spell vamp.

TIPS:
Try to abuse your passive and hit as large a minion wave as possible.
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Ahri's Q is a skillshot called Orb of Deception
Range: 880
Cooldown: 7
Cost: 70 / 75 / 80 / 85 / 90 Mana

Active: Ahri sends out an orb in a line in front of her and then pulls it back, dealing magic damage on the way out and true damage on the way back.
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Ahri's W is Fox-Fire
Range: 800
Cooldown: 9 / 8 / 7 / 6 / 5
Cost: 60 Mana
Active: Ahri releases three fox-fires to surround her for up to 5 seconds. After a short delay after cast, each flame will target the closest enemy unit to itself, prioritizing champions, and deal magic damage to the target.
Additional fox-fires that hit the same target will only deal 50% damage.
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Ahri's E is a skillshot called Charm
Range: 975
Cooldown: 12
Cost: 50 / 65 / 80 / 95 / 110 mana
Active: Ahri blows a kiss that travels in a line in front of her. The first enemy it hits takes magic damage and is charmed, forcing them to walk harmlessly towards Ahri while being slowed by 50% for the duration.

Ahri's ultimate skill is Spirit Rush
Range: 550
Cooldown: 110 / 95 / 80
Cost: 100 mana
Active: Ahri dashes towards the cursor and fires essence bolts, dealing magic damage to up to 3 nearby enemies, prioritizing champions. In the next 10 seconds, Spirit Rush can be cast two additional times before going on cooldown. Each enemy can only be hit once per dash.

Cassiopeia
Difficulty: Hard
Cassiopeia is a very pokey champion and is very annoying early game.
She can basically out-burst you if you aren't careful enough. Get an Abyssal Mask.
Cassiopeia's Skillset:_____________________________________________________________________________
Annie
Difficulty: Easy
Annie is a very annoying champion early-late game and head on.
She can out burst you. Use your Charm and Zhonya's Hourglass to your advantage.
Tips: When there is a grey circle around her, her stun is up. BE CAREFUL!
Annie's Skillset:_____________________________________________________________________________
Vayne
Difficulty = Medium
You rarely see any mid lane Vayne's anymore.
Be careful of her Final Hour + Tumble Combo = Invisibility
Vayne's Skillset:

Ignite:

Ignite is a good choice for Ahri, It allows her to pick up easy kills.

Flash:

Flash is also a good choice and is viable as it is on about 99% of all champions. It can allow her to chase/escape and go through walls.

4/20/13 Guide First Made
4/26/13 Updated a few things.
4/28/13 Added New Build
4/29/13 Added a Summoner Spells section and fixed some typos.