Kennen, well not much to say, amazing harasser, is a very powerfull nuke, has his own escape mechanism, fun as hell to use, excellent farmer (to a point) and hes extremely fun to play :D

Pros
Epic harasser
Extremely powerful attacks
He's a NINJA!
Gets a stun at lvl 2
No mana

Cons
Squishy
Energy Bar instead but it doesn't take much to recharge
His main escape mechanism takes a good chunk of energy

Kind of a given, more ap= more dmg, more mp= more dmg, dodge= less getting hit which is always nice

The shield gives you extra survivability which is a good thing, the boots... well more mp hence more dmg, mejais (cant stress this enough) gives a hell lot of ap and as kennen, well the more the merrier, Rylai's because the HP and AP help a lot and the slow is extremely usefull and just helps even more in harassing others.

I left out the last 2 spots because the items can be situational, I personally would go with Rabandon's Death Cap because your Ap soars with it, and Zhonya's Hourglass because the defence in general will save your life unless the other team is completely stacking AP but its not your job to tank so its all good, and the ap just adds on to your crazy damage, however if the enemy team is major stacking magic defense, the obvious choice is void staff!

your shuriken is powerfull, use it to last hit minions and eventually you can combine lightning rush and electrical surge to take out complete waves of creeps.

Never initiate a team fight
In general you can take out most hero's 1vs1 because you can keep them at arms length, more importantly shurikens length
Hit them with your shuriken, run trough em with electrical surge, then to finish them off electrical surge, if they're not dead by now i would suggest using ur shuriken again, still alive? hit em with your hextech gunblade, if you still havent finished them off they're either a tank or, well practice makes perfect :D

First time as a guide, i appreciate good and bad criticism so leave a comment