This build is for the people who want a viable alternative then just AP Katarina. Please try the build first before saying anything. You can play mid or top as you want.

I focus on early AD & late AP. Skill Q, good farm & good damage. Don't forget you have a strong auto attack. After you Q + E you can further harass due to high AD.

Reason AD? Q and R scale better with AD then AP. You need at least a Gunblade for some AD. I've seen kata's with Wota(gunblade & wota might work) but only wota is fail imo. I really like this early AD, i hope u will see it too.
You can play with AP or AP/lvl and MPen runes, but get the pickaxe before the revolver, it is just so nice :).

If you have spare money buy a ward, or 2, so u can escape all the time (place over wall + shunpo, smartcast is advised).

Very important is 'Mercenary' in the defense tree. U know Kata's passive? multiply it with 2.

In this build you NEED Sorcerers shoes, since we have no MPen runes. But when u have them, damage will be pretty nice.

Push your lane (twice Q) and gank a lane. Should be pretty successful. Care for countergank from mid, or if u know so, intercept him.

For an extensive guide on Kata pls go to http://www.mobafire.com/league-of-legends/build/your-ap-katarina-du-couteau-resource-dont-play-with-knives-61072

Core build: Sorcerers shoes - Hextech Gunblade - Rylais scepter - Rabadons - Void staff - and some defensive item u think it is best for that time (i don't need to hold your hand don't i). Don't blindly follow it, just keep in mind these are the items u need to get, the order doesn't really matter

And don't forget to use the active on Gunblade(300 magic damage). Great for chasing, bursting, and even to finish off the last guy to get your penta ;).

Please comment & rate. no trolls pls!