First of all, even though I am not a "high-elo" summoner in League of Legends,(because I am only lvl 25 at present) doesn't mean that I didn't have the experience.But actually, I have another account but then.... I DON'T FREAKIN' KNOW THE PASSWORD!(1 point for you brain..)

So anyway.... for Katarina, I would really tell you not to pick Katarina if you are not going to be in the middle lane or like you have Lux , Fizz , Akali , etc. because they will and will be placed in the middle lane.

Pros:
-Has a lot of AP and AD early until late game.
- Has 2 escaping abilities. (Shunpo and Flash)
- Has great farming abilities
- NO FREAKIN' MANA!
- Good Choice for Ganking
- Shunpo can be used for killing or escaping

Cons:
- Tends to get focused a lot....
- Ult can be interrupted
- Ult covers short AOE

You need to have 9 Mark of Magic Penetration because you are an AP champion and you need to deal significantly heavy damage for your team.

Also you need to have 6 Seal of Scaling Health so that you will be tanky for you will have 2000+ health. But then I also recommend you pick 3 Seal of Ability Power for the bonus AP.

And also when you are laning in the mid, you shall expect to have an AP champion to be your enemy there. So I recommend you to have all 9 Glyph of Magic Resist to negate or lessen the damage of there spells.

Then you will need to have 3 Quintessence of Ability Power for of course the bonus ability power.

Voracity
Katarina's passive is really really nice. It is usually good for her ult because you could cast it again for only 20 sec. if you killed somebody. Because if you could kill or have an assist for a kill, Katarina's spells are instantly refreshed and her ult's cooldown is reduced by 10 seconds. And it also stacks for the blue buff that you should get every time it spawns starting when you're level 6.
So overall, a great passive for Katarina!(Hands up for RIOT!)

Bouncing Blade

This skill is like a multi-tool. It could be used for harassing and farming. It is also good for mid-game damage when you have a Rabadon's Deathcap and a Sorcerer's Shoes in your item build. When your team is clashing, try to cast this skill in the enemy champions then use Shunpo to there AP or AD carry then use the sinister steel for maximum damage. And also when the team has or is in the middle of a clash, use Shunpo in the middle of the fight then use your ult Death Lotus.

sinister steel

This skill is amazing when you are harassing AD champions like Jax or Ezreal . Because when you cast your first skill, you would want to use Shunpo to him/her thhen execute sinister steel.

Shunpo

Now this skill is so good and fitting for Katarina because she is an excellent ganker and also an assassin. Try to use Shunpo when you are chasing down a kill or when your health is low and is running to escape.

Death Lotus

This skill is so AMAZING! Paired with 2 Rabadon's Deathcap and a Sorcerer's Shoes, you are most likely unstoppable with this skill. You should use it in team fights to increase your chances of having a double kill, triple kill, Quadra Kill or maybe a PENTA KILL! No really, I am not joking because I experimented with this skill and it is most amazing in team fights.

Thanks to Bouncing Blade and sinister steel, you will have a nice combo for farming.
(COMBO= Q+E+W) You need to use Shunpo on the creep in the back(the long range one) then use your sinister steel to finish them all off.

Well I know that you wont trust me if I said that I have had 2 Quadra Kills in one game playing as Katarina.

Well guys, this is the proof......

Also this was the second time in the game.......

So I wish that you enjoy what you read in my build and I hope that you comment and vote everytime you read my guide. But I know that it is not as good as the others or as much informations like the other one's, I hope that you learned something on Katarina and I sincerely hope that Katarina would be your Favorite Champion after you read this. THANK YOU and GOD BLESS!