My summoner name is Errzza, i have been playing LoL since the beginning of season 2 and Katarina was the first champion I bought, I am here to make a guide on how to counter every champion you will face during mid lane, and make your build successful against anything.
Never had a penta? Here is your chance.

Fore upgrading here is how you want to upgrade:
R>Q>W>E
Your skill sequence should be this (After the ')' is if they are still alive)
Q -> E -> W -> R.). -> W -> Q -> E -> W -> Q

You want to have your tank make the engage, then wait about 3 seconds and if a lot of their team is at near half health, its time for you to combo in for the free triple, following with a quadra then penta.

PROS:
High mobility (with shunpo and 'W' speed bonus)
Great escape (ward hop, minion hop)
AMAZING burst damage
Sexy skins (Those pixels!)

CONS:
Often focused with heavy CC
Dead quickly, without Rylai's
Weak Pre- lvl 6
Banned 40% of the time in draft/ low elo.

If you are facing a champion in lane that has a lot of range over you, or can frequently spam abilities, then you can last hit with your Bouncing Blades (q), but try to get as much last hits as possible without getting poked, Your life is more important than 1-3 CS.