Kennen Is home to many players, and many different play styles This build is only for those summoners who enjoy high-risk high-reward play. In other-words, Very very aggressive.
This guide is for Hybrid Kennen 1v1 top, and Hybrid Kennen 1v2 Bot.

I Love kennen, one of my first pickups as a new summoner and what a great pickup. And he has fallen into a place where he can be used to counter that pesky red pot start top, or that obnoxious blitz/thresh grab bot lanes

This is what i use, there is a large amount of leeway. Use what feels right; I have found this to serve me well.

This item build is a nice mix, with a large amount of magic pen and %physical damage.

Botrk provides a nice nuke to follow up your own stun or make it easier to land.

Liandry's Provides nice aoe damage on your E, W, and R, While giving your Q decent poke with Low AP.

Guinsoos Gives your ult a sort of OH ****! feeling, if you are below50% you get spellvamp and attackspeed, allowing you to BOTRK back up while vamping from your CC.

Lizard adds some true damage on top of your already high Pen build, but this item can be Changed with many others, Personally i think the only other viable options are DFG and Rylais, and maybe a Frozen Mallet.

Extra space, I would fill it with whats suits the situation. Typically Frozen Mallet, DFG, Rylais, Guardian Angel, Ruaan's Hurricane. You need to use your critical thinking here, What does your team need? What do you think would help out most during a teamfight.

1v2 Bot, This occurs when you want to shut down there red pot starting top lane. You grab boots 3 pot, and utilize your q and e for safe farming.

1v1 Top, be very aggressive. Make them use potions early, make their jungle come up top, and simply use your e to remain safe. You must be wary, be aware of your and their capabilities at any point in time, warding is a must. This play-style will encourage and enable your jungle to gank the other lanes while their jungle is top. a word of advice, BE aggressive, but never dive towers, and never fully commit to a fight that you do not know is 100% yours, so in short Be aggressive, but don't be stupid

Pros:
Incredibly mobile and safe champion
Decent Poke
Able to 1v2
No mana/High sustain
synergizes will with Magic pen armor pen, and life-steal/spell-vamp
Amazing ult and utility for a high damage champion
Very difficult to zone
Able to build many ways

Con:
Reliant on Team followup
Countered by high burst
all skill shotsb