good build and guide... but one thing....... MY PIKACHU EVOLVED INTO GRAGAS.

Good overall guide. My only suggestions is to go more in-depth about his abilities and combos people can use to activate his passive. Also, a minor problem in your masteries: you take improved ghost but you have flash and teleport as summoner spells :D

Do not get abyssal scepter and void staff. these items do not work that well together. THe base amount from void staff, boots, and runes gets applied before the percent from void staff, so the 40% isn't doing much good. I would recommend a lich bane instead of void staff.

I run a 9/0/21 because he could use the reduced summoner spell cooldowns. I would prefer having an almost guarenteed escape plan (Flash, Lightning Rush) than to think that Kennen is tankier than he is, and over-extend and die.

Okay, Clarity is obviously not a strong choice haha I see where you are getting at. But revive isnt half bad, seeing how good of a creep wave killer he is. A lot of pros are using revive now because its seeing as giving you 50 seconds extra to save your nexus for a possible comeback win. Trust me, ive seen it happen. Just wanted to point that out haha. But yeah, I wouldnt use revive on him just because its not my style :D
Why do you use a 9/0/21 build? 9/21/0 build is much better in my opinion. He IS a caster but he does NOT use mana or need the extra things that utility gives. I'd much rather be harder to kill. I gave your build a +1 for the fact that it's a good build, but I'd like some explaining haha

Yeah, I tend to grab the Abyssal after my Zhonya's, so my ulti does tons of damage.

i dont normaly pick off mejis, but thats my own choose, gruid build and guide man, u ever tried abyssal ? its a great item on kennen!!

Look like a pretty sturdy build... ill try it out for my next game