i use this kennen build cuz its the best build you can get for kennen.
kennen is a ad carry who can stun so with a nice cc.
thats why i usualy like a blitz or alistar support next 2 it.
u go for the early lifesteal 2 be more sustain.
and the early atack speed is that u can get ur marks on them faster and stun more.
and then the infinity edge is of course for more dmg.

then usually i sell 1 of my dorans blades 2 buy a guardian angel so it will be harder 2 get killed.
so then they usually go and make u 2nd target and take out the ap mid 1st.

i like this champion alot in combination with fiddlesticks for the double ult.
kennen's ult stuns people in a big aoe circle and fiddlesticks deals tons of dmg with a same size aoe circle

ad carry kennen will rly have a nice early game dmg if u use the runebook i use on him
you could also go for more armour pen instead of the atack dmg runes.
and you also could go for atack speed marks and armour pen quint's for more stuns.