AD KENNEN IS HUGELY VIABLE.
80 games won in ranked with him (75% of games played with AD Kennen)
boots + 3x health pots.
Then you need to build based on what their team is building. I usually rush Madred's razors (for farming) then go Phage, Zerker's greaves and finish Madred's up. Then wit's end, finish Frozen Mallet and then build a Phantom Dancer and an IE.
Skills: W,E,Q,W,W,R,W,E,W (choose on your own from here on out.)
Don't mid as AD Kennen, try to get bot with a support. Taric, Nidalee, Nunu, and Sona are all great supports for him. Try your own play style as well. Your E is usually used as an escape or an initiate, your q is to stun stack and tag opponents from a distance. And your ult is for team fights to disrupt and 1v1's to gain distance or initiate. or escape from other situations. good luck and happy playing sir.