Not Updated For Current Season
One of the best champs in the game, she has very strong skills and can easily carry a team. If Kat is not targeted down or cc'd to death she can and will win team fights for her team. Kat is a mid champ plain and simple she really has no reason not to take mid except for a few that I will mention later. She is a great harasser a great nuke and a great assassin aka nuke/ganker.
She is by far my personal favorite champ some of the scores that I've put up with her are 17 0 3 and 18 4 15 if you play smart you can and will die less and kill more just because that is how Kat is.
One thing I feel compelled to let everyone know you need to be confident with Kat for any of this to work if you aren't confident then don't mid you will ruin games by losing/feeding mid.
Greater Mark of Insight x 9
The Greater Mark of Insight provides us with 8.55 Magic Penetration to start the game with. This is great for any Magic Damage Dealer because it allows you to deal greater amounts of damage to your enemies early on in the game and it helps during the late game phase as well. Overall, a good rune choice in my opinion.
Greater Seal of Evasion x 9
The reasons for why I choose to use the Greater Seal of Evasion is because I do not have any items that greatly increase survivability. These runes provide me with 6.75% Dodge Chance which is not much but it is definitely something that can save you. If you disagree with the use of this rune you could definitely use a different one.
Greater Glyph of Focus x 9
The Glyphs of Focus provides us with 5,85% Cooldown Reduction, These allow you to use your Q and E abilities more freely, as well as those fun Double Rainbows.
Greater Quintessence of Insight x 3
Greater Quintessence of Insight these give you additional 5,67 Magic Penetration ontop of the tons you will have in each build, and help you quite a bit early since you don't get much AP until Mid game
For Masteries i run a 9-21-0 Pattern, getting a full defensive tree for the health, ap % and damage reduction. That AP% stacks on top of your Hat, giving you 34% bonus ap at level 18. In the offensive i get the exhaust buff, the ap/level spec, CDR and the magic penetration spec (percentile penetration is always very good to have)
:The reason for why I used to chose to use the Sorcerer's Shoes is evident. 20 Flat Magic Penetration will help you rip apart the enemy team. The Sorcerer's Shoes were definitely the best boots for Katarina unless of course the enemy team has a lot if Crowd Control, in which case the Mercury's Treads may have been a better alternative. I now use the Ionian Boots of Lucidity.
:Please don't hate on snowball items. At least for Kat, this item is hella necessary. Nothing else at this price (let alone most other items) can provide this much AP. Plus, with the deathcap, not even including the bonus % AP masteries, the total 180 AP from this item with full stacks turns into 234 total AP. And getting stacks, especially with Kat, isn't as hard as it may seem. Your abilities are so devastating early through late game that you are guaranteed to get a lot of kills and assists. And if you loose some, just get them back! If you spin with your ulti in a teamfight and your team winds up winning the fight, you will either get 6 stacks maximum if you get all 3 kills from the spin or 3 minimum if they all turn out to be assists. A few teamfights, some ganks, picking some people off, and you have your 20 stacks. Not only is the AP from this item amazing, but once you get full stacks, you get 15% CDR, which, with your ionian boots and CDR glyphs, will let you Shunpo, Bouncing Blade, use Killer Instincts, and ulti whenever you want, almost instantly after you just did, even if you haven't gotten any kills or assists for a while (for your passive). No other item could provide all of these vital benefits for Kat, especially at such a low price.
:This item is great. It gives you a great amount of AP and HP, both needed for Kat, as well as its passive. Whenever you cast a spell on someone, it slows them by 35% for 1.5 seconds (or 15% for a multi-target spell such as Kat's ulti and Bouncing Blade). What does this mean? It means that if you Bouncing Blade a group of enemy champs, they will all slow down (not much, but it's something). This slow helps you to escape or chase enemies, as well as help with your ulti. How? If you are chasing someone, when you Shunpo to them, they will get slowed, and by the time the slow wears off you will have your Shunpo back in around 1 second. This means that from that inital Shunpo-slow, they will be guaranteed to stay in range of your next Shunpos until you kill them. You can also use a Bouncing Blade on them to slow them again when your Shunpo isn't up. As for escaping, you can pop a quick Bouncing Blade on a pursuer to slow them for a bit, which can be just enough time to Shunpo or flash out of the situation. How does this all tie in with your ultimate? The slow of the ultimate will keep the enemies you are targeting in range of it the whole time. See the "Skill Combo" section of "Skills, Skill Sequence, and Skill Combo" for more information on Rylai's with your ultimate.
:This item is just amazing. It adds 155 AP plus 30% of your total AP. So, by the end of the build, this item grants a total of 330.5 AP, if you have full Mejai stacks, not including masteries. This is with the More Tanky Enemies Item Build; for the More Squishy Enemies Item Build, the total AP granted from this item, not including masteries, with full Mejai stacks, is 324.5 AP.
:This item is great. It adds more AP, and gives you armor as well. This armor is terrific to reduce damage you might take when channeling your ultimate, and works great with the reduced damage you take when you Shunpo using Killer Instincts. As well, the active is terrific when against enemies such as Karthus (his ultimate), especially if you might die from it, as well as AoE and CC ultimates such as Kennen's and the second burst of Morgana's ultimate (to prevent the stun). It also works great when you are using your ultimate in a big teamfight, you can pop the active once you're done if you are taking heavy damage, to let your teammates come and push them away from you. Great item for Kat :)
:This item is great against more tanky enemies who have a lot of mres. It penetrates 40% of their mres, as well as grants 70 AP. The AP is great for you, obviously, and the mres, in addition to the Marks of Insight and the Archaic Knowledge mastery point, will let your abilities slice through everyone, even tanks.
:Well, if there is someone escaping you, and they seem to be faster than you, Shunpo, Exhaust, and then let your frenzy begin. Keep shunpo-ing to them, use your ulti, whatever you want to do to assure that they have a bad day.
Similarly, this is great to keep people in your ulti. Pop it on someone you think might get out of range when you spin in a teamfight or if you are ganking or even just picking someone off, and then you can spin with a relaxed state of mind knowing that they aren't going anywhere anytime soon.
:Great for finishing off enemies who you don't feel like shunpo-ing to to finish off (if you are both low health and they are escaping to a tower, early game when a tower can two-hit you, if you don't want to leave a teamfight, etc.)
:Why flash, you ask? Why would you use flash, wRAth, when you already have a crazy flash ability known as Shunpo that has MORE range than the flash? This is why.
There is a place known as the jungle. Now, what might happen is you are strolling around...and OH NO 5 people come to try to screw you up. No ally or enemy units nearby...but you have flash. Flash away, and if you happen to flash in range of a friendly or enemy unit, you can shunpo to them. Happiness!
There are a few people turret hugging that you wanna screw over so bad. They are JUST out of range of your Shunpo and there aren't any minions nearby. Flash in, shunpo to them to get closer if need be, and spin to win. =D
Flash can also be used to escape with your teammates, if you happen to be the laggard and your teammates don't have a soul to stop moving for one second so you can shunpo to them, flash near them and then shunpo.
You can also pull off a classic Fiddlesticks ulti kinda thing. Hide in a bush or somewhere in the enemy's fog of war, and if nothing is in range of your shunpo, flash in, shunpo if need be, and just like in the turret hugging example, spin to win.
So yea, flash helps. A lot.
Pros / Cons
Shunpo - Most Versatile Ability in the GAME
Not hard to learn
Amazing mid-late game farmer
French and Sexy
Not a great EARLY game farmer
Easy to counter
Death Lotus is channeled and is prone to interrupts
Although Kat isn't the best of farmers early game, just make sure you stay back and last hit whenever possible. If a wave of minions are low on health, pop a Bouncing Blade on them to get a few last hits. Mid-late game is where you shine with farming. Whether you build AD or AP, your Q can annihilate a wave of minions. Just pop it and you will get around 150 g in last hits.