So, There are pros and cons for each champion
I know you can figure it out for Kennen, But hey im nice.

Pros:
Great farmer mid game
Extreme harasser if your good at skill shots
Uber huge AoE CC and single target CC
If played right you will almost never die
Lots of escape abilities.


Cons:
Squishy early to mid game
Prone target in teamfights (is not a problem unless you dont have your ultimate)
Extremely item heavy, So you NEED kills or alot of minion kills to support your damage to its fullest extent
Hard to know when to retreat if your ultimate is on (People like to over-stay then die)

Okay, So you locked in Kennen, You want to know how to play him, You complain about bad teams, i don't know how to build. ect. Now this build just revolves around me and how I play. It may not be the top choice for you but you might like it.If you don't like the guide, move on. Now.

Okay, lets talk first about mastery. I go 9-11-10. I pick up archaic knowledge (obviously) and get defensive mastery because Kennen is EXTREMELY squishy with this build. Well, squishy until you get to late game. I take points in Good Hands because a Kennen with a 1 minute timer can't stun at all. Best take the 50 . No. Yes? If you said no you're an idiot. Anyways lets head over to the runes.. I take Magic Penetration marks just...because Kennen is an AP caster. I take magic resist seals because no one will get to you unless your 2v1'ing. Like i said, Don't attempt it. Or if you have a really bad laning partner, or the enemy has CC....So..The only thing able to reach you usually is the skill shot or spell. So i take flats. I take flat cooldown seals or flat ability power. The choice of Quints are all up to you. Though i would prefer flat HP.

I take Flash and Ignite for my summoner skills, Flash to position myself for Maelstrom, ignite to finish off people like tryndamere and the "I CAN DO IT" tanks. You can take ghost if your having trouble running away or your always gank prone. Just dont take fortify or revive...I dont have to explain clarity at all.

Okay, You might call me a ****** for starting with Dorans Ring but the reason i chose this item is because of the Health and the Ability power. Same cost as Doran's shield but its so much better (You dont even need the armor until mid to late game, by then it will be useless).
My first B you should ENOUGH money to get boots of speed AND a AP tome. Or just boots of speed and healthpots. I build my boots of speed into Magic Pen shoes because ... well you bring CC. Even if your susceptible to it i won't recommend it. You can though. I dont care. I build my AP tome into a Mejais if your getting yummy kills (At LEAST 3-0-3 to get it) or just build a Haunting Guise.


Mid game i would have 10 stacks , so i buy my DTG or just build up a Rylai. Can go either way. 10 stacks is enough for you to 1 hit minion waves with E>W combo. If you dont have the stacks go the Rylai way.Now if you had 10 stacks and now have archangel's you can grab your Big Stick the Large Rod, and buy Rabadans the Death Nuke. Or start on your DTG if you didnt get the yummy stacks early game. Obviously finishing up Rabadans after if you didnt get the yums. You can replace dorans ring if the game is THAT long with a Zhonyas, But personally my Rush is all it takes to run away.

Early Game

Your early game if you did not know is where you farm to be a fat and chubby Kennen. To do this you must have a baby sister. Some are listed below:

Sona
Kayle
Soraka (Though her infuse is almost useless, still good for silences)
Janna (Extreme CC plus Extreme CC.Yea.)
Sion (Look above)
Tryndamere
Ashe
Basically almost all the AD carries because top will be raped with your stuns.
Just dont overextend

In early game you can over extend freely if your dominating whatever lane or in. Keep an eye out though if the enemy has stealth like Roamelynn or Twitch the Pesky Ambusher. But your stun is enough. Right? =). Focus on last hitting and harrassing. By level 3 your rotation should be E>Q>W or E>W>Q Then pop your ult if your level 6 and they are half hp.Finish off with ignite



Mid Game:
Focus mainly on harassing from here, Last hits are always welcome
Try and nab kills with your best friend CC or stay passive and defend.
If you follow the Mejai path and you have no stacks, TRY TO GET A KILL BY GANKING
I can't stress this priority, if you thought you could get 10-1 or a score like it and you don't you just threw a grand down the toilet. A GRAND. So if you DONT have stacks or if you need more gank. But if you have yummy stacks (over 5) stay turtle so you dont lose your tummy yummys.



Late Game:
Team fights will be taking place here. If not your priority is farming. Still. If you dont have any stacks for mejai yet this is where you'll be getting them...But its a tad too late. Still. AP more powerful than your rage cap is nice. Right? Here youll have your ulti slowing by 15% and massive stuns each 0.4 seconds your ulti dealing over 400 damage! couple this with your nuker the Q and your chaser the E. Your guarenteed a double kill. If your behind your pretty doomed. So farm as best you can before your team calls you in again to stun the **** out of the enemy Miss Fortune or Ashe. Late game you can switch out your Dorans ring for Zhonyas or Abyssal scepter. All up to you.

Hexdrinker: Get this if the enemy team in mid game is all casters. WILL save your life multiple times.

Abyssal Scepter: I would rather have my AP support like janna get this...Yea even then its sad. And its sadder that i would rather have a Janna use it than Kennen. Even though it may prove useful...1/10000 of the time -.-".

Death Fire Grasp:
This item is part of the Core Build...But it can be situational. Use With Caution.


Randuin's Omen:
You can be a super tanky i stun u Kennen. If your team doesnt have a full fledged tank

There are more, and its up entirely to you to choose them. But i'm pretty much done for now.

So...Thanks for reading. This is my first guide. I applaud those of you who read to the end. I wish i could give you a sticker or something =/.