Hello and welcome to my first League of Legends guide ever. After being able to study the recent metagame after it has been changed with new masteries, new summoner spells etc I was happily surprised to see that recently unapprecatied mid lane malee champions like Kassadin or Katarina are sure to get a lot of attention in the upcoming season. To celebrate this, with Kata as one of my beloved champions (love trolling opponents with her), I decided to do this build. I have played numerous games with her and am sure to play her more as an AP carry mid lane. In this build I will show you what's my strategy, how I counter my opponents and in what way I can help my team in late game.

As with many builds published on this site, please do not pay much attention to the items mentioned in the cheet sheet. Of course, they aren't taken from void, but if you read this guide you will see that my item choises and their order may vary very much depending on the situation in game.

For those who often find themselves disorientated in the vocabulary of a "nerd" League players.
AP - Ability Power
AD - Attack Damage
CC - Crowd Control (stuns, slows, fears, etc)
MR - Magic Resistance
HP - Health Points
CDR - Cooldown Reduction
MP - Mana Points
TF - Team Fight

Pros:

+ works without mana
+ very sustainable early game
+ hard to be ganged
+ great escaper
+ amazing burst damage from ultimate
+ amazing cooldowns in late game

Cons:

- not great farmer early game
- vulnerable to CC
- she can't move while ulting
- her ult can be easily stopped
- highly focused in late game

Voracity (passive)

One of the best passives in game, which makes you extremely useful in team fights. Whenever you kill or assist with killing someone, your cooldowns are reduced by 15 seconds and you get extra gold. This means all of your 3 basic spells are available instantly and your ulti is really close to load, so you can burst again. Just let me tell you that if your team manages to get 3 kills with your ult on, you don't have to wait to beat the other 2 luckers with the same ult down to the ground!
Use this for: cooldown reduction.

Bouncing Blade (1st maxed spell)

This is your most effective and overlooked weapon while dealing with mid lane. So, when you should use it? Everytime. Simply spam this spell just when your cooldown is off and you see your target. Why? Cause you are Kata. You don't have mana and you don't care.
Since this is the spell you should max first, I feel like I should provide you with some argumentation why I recommend this, especially since I often see some Kata's builds that suggest maxing Shumpo just because it is such an awesome spell. However, even though I agree that it is really great one... It simply doesn't need maxing. It's just like I said max Vlad's W since it's so awesome and it let's you get away from any damage. Your blades are what truly makes you deadly early on. You may be a little negatively surprised to see that they aren't as... bouncing as you thought, but this is until you give them some levels. They are somehow similar to one of Sivir's abilities - the higher their level, the more they bounce and more damage they make. It's really awesome to be able to safely hit an enemy champion without getting close to them, but just by hitting minion wave that is just passing by. And, this being said, remember that this skill is extremely good for farming. It may not be very effective from the very beginning, but wait till it's somewhere on level 3 (your champion would be on level 5 by that time) and it will start to do some serious harass. You'll also find out that this skill is very viable once you'll get your Hextech / Will of the Ancients - the amount of health you can spell vamp with this may really be outrageous.
Use this for: farming, harrasing, enjoying free health with your spell vamp.

Killer Instincts (max last)

This spell is one of the "double-passive" kind of spells, and just like most of them I decided to max it last. The first, always active passive, gives your auto and magic attacks a low amount of additional damage, which is always nice, and the activated passive is used for performing different versions of your two other basic spells.
If you use it for your blades, the damage they do to subsequent targets will increase from 90% to full 100% and they will get a 50% healing reduction for 5 seconds. Enemy champion may find this really annoying if they have a spell vamp / life steal, or are in the middle of drinking a health potion.
This spell used to be very rarely used for Shunpo whatsoever, since it only gave it some low (up to 35% when maxed and only for 3 seconds) damage reduction to the enemy target. Since the season 2 patch is on, using this spell also determines your location, whether you will appear in front of the target or behind them. I myself find this a very minor change, but keep it in mind - if you happen to be chasing an enemy with Shunpo and your killer instincts is available to be used, it's always worth doing it for chasing purposes.
Use this for: better farming, reducing your opponent's healing (with Bouncing Blade), appearing behind your target (Shunpo).

Shunpo (max 2nd)

The second offensive spell of Katarina is also a movement spell. It lets you teleport to selected target (including champions, minions, visible wards, pets etc, but not any buildings) and, if they are your enemies, you also do some nice damage to them. This skill has a pretty big range (not as great as your maxed Bouncing Blades though) and can be used for both escaping or chasing an enemy. You can also find it an option for harassing your opponents in lane phase, if you feel confident enough. Please keep in mind that you aren't Jax and doing this spell would not affect your enemy with any CC and even Killer Instincts is not really a great shield for you. However, this is still a great spell and it has its uses even early on, such as spell vamping or getting easy last hits. This also gives you range you need while being malee.
Keep in mind, that even this is more of movement spell, it can do really big amounts of damage later on and you can use it to quickly knock down enemy champion with low amounts of HP going on. If you need, use a flash to increase your range even further. By the way, this also can be used as an addition flash.
Use this for: last hitting, spell vamping and harrasing (early game), get in team fights, escaping or chasing (late game).

Death Lotus (ultimate)

And finally, up to her final weapon of destruction, a double-edge sword, Katarina's ultimate. First of all, it's a very controversial spell and its usability highly depends on your and enemy team. While it surely does great amount of damage and is great for doing triple, quadra, penta kills etc, on the other hand... Well, let's just say that the main problem is that any movement of your own while you do this ulti instantly stops it, just like with Miss Fortune. That means that your enemies has no reason to stay close enough to you to be able to take further damage from it, and also nothing prevents them from stunning you (and stopping poor ulti) when you do it. That means, if you don't have good CC, or are fighting against good CC, you aren't in very good situation. If both happen at one time, then... Let's face it - you are utterly useless for the whole mid/late game.
Use this for: farming... Farming champions obviously.

I wouldn't like to get into much debate about you spell possibilities, since it's mostly choice of your will and your personal game style, so I will just quickly describe and justify choices of my own.

Flash

While many people did took Flash for granted in season 1, I don't think it would change very much even with the nerf going on. The most common reason to take this spell is just because others will have it, and since they have it nerfed as well, nerf of your own one isn't a big pain. Also, it combos well with your Shunpo, letting you cover the range you need to quickly shunp to your opponent or your own ally or any minion while you escape.
Use this for: chasing & escaping.

Cleanse

Cleanse wasn't a very popular spell in s1, but I still found it very useful back then. And now, with enhancement of being able to stop any exhaust or ignite it is a really powerful helping tool. Just think how many times you can easily find yourself being killed just by one stun and getting powerful focus from the whole enemy team shortly after it's been landed on you. Being able to get out of any CC or occasionally saving yourself from 500 damage Ignite is just splendid. However, beware: with this setup, both of your summoner spells have high recharge, so taking 21 points in utility mastery is a must.
Use this for: escaping CCs / ignites if you see instant death.

Other possibilities

Exhaust

Many people suggest this, as this is free CC to combine with your ultimate, but let's face it this way... If an opponent has a stun, he will still stun you, but with less damage. If he has flash - he will still escape with this on. If he has Cleanse (rare thing) - don't even discuss this. Otherwise, there are much better CCs build-in other champions. If your team sees you pick Kata, they should think of taking one of that champs.
Use this for: utilizing your damage while doing Death Lotus.

Ignite

This spell was always helpful for getting free kills with it's high damage, but I don't feel like Katarina is one the champions that needs is so much. If you need healing reduction, just use killer instincts for your blades. If you want high damage, do ulti. That's pretty much it.
Use this for: getting free damage to kill luckers.

Teleport

As a mid char, you may think of using this one, but unfortunately you are basically not the pusher in here and you rather won't find yourself needing to recall very often. Your lane sustainability is very high in this build plus having no mana makes no way for you to need to recharge often enough for this spell to be utilized well.
Use this for: faster come back after recall / doing gangs on other lanes.

Simply forget about other spells. Just on rare occasions, if you find yourself playing without support in your team and no one from your team has room for additional spells, you may consider taking CV on your own, instead of Cleanse.

Masteries

I feel like these two doesn't really affect your gameplay in really major way, but surely in season 2 masteries play bigger role than before.
Some people may consider doing 9/21/0 mastery tree if they feel like they need more defensive measures, but I myself found it very important to get improved summoner spells reload and free spell vamp. Those tho are extremely useful masteries on Katarina, for which I'm ready to forget completely about the defensive tree. However, if you need more survivability, you may do classic 9/21/0 as well. Just remember not to change that offensive tree - free AP, cooldown reduction and magic penetration are all really important things for Kata.

Runes

My rune setup is pretty straight forward and is centered around being more effective in early game. With my Runes and Masteries, you will start with 28 AP with no items, which is pretty solid base for your spells. Magic penetration is always useful for doing more damage to champions, and Armor Seals reduce your damage against minions or evil junglers coming out from nowhere (unless they are Fiddlesticks).

Starting items

By grabbing Boots of Speed and 3 Health Potions, you may find yourself very comfortable on the beginning. With enhanced movement speed, you will be able to safely dodge most of your opponents hits, and if you get damaged or ganged, potions will prevent you from getting back to your base every now and then. The other possible way to start this build may be by taking Amplifying Tome and 1 Potion, but I like the mobility early boots give me, plus just 1 potion may not be enough. However, if you want to go more offensive (which is not really advised on early game Katarina), you may consider starting that way.

Core items

As soon as you get back to base (which is not likely to happen very early), you must make sure to get all the items you will need for mid game. That being said, you should instantly pick Hextech Revolver - Spell Vamp is extremely important for characters like Kata. Your boots should be no else than Sorcerer's Shoes. Although I found builds with Ionian Boots of Lucidity, I don't think you will need further cooldown reduction than you already have from masteries and your passive. Sadly enough, this seems to be the only available item for CDR in AP Kata build, so if you really need this, you can pick that boots and get more magic penetration later on from other items like Void Staff. As you will soon find yourself getting full Will of the Ancients from your Hextech, you'll have a choice of either getting an early Rylai's Crystal Scepter or rushing your Rabadon's Deathcap. If you feel safe and your team has already some nice CC, you can invest in more damage with Rabadon's, but if you feel like you need more survivability, Rylai may be better option. However, in some games, if you're doing well enough, you may not feel like building Rylai at all, which is not really recommended, since its passive really combines well with your ulti and prevents opponents from fleeing from you. Needless to say, Rabadon's is really a must in this build, so if you decided to do Rylai, grab it as soon as you finished with previous items.

Further Item options

There is no real one-way path for your father build after you have your core items. Only one thing is sure: you must improve your ability power and meanwhile get other stuff you feel like you need.
Zhonya's Hourglass is great choice if you need more armor and your opponent's stuns get so annoying that Cleanse is not enough for them (or perhaps you don't use Cleanse at all). Besides, I really recommend taking this item when fighting against strong AP nukers, like Karthus, Fiddlesticks or Veigar. Being able to negate their ultimates completely is just like a bless.
Void Staff is a cheap late game item that will help you to get through more tanky opponents that are stacking magic resistance. Plus, if you took CDR boots instead of standard mage ones, this item is highly important for you to get as fast as you complete Rabadon's.
Abyssal Scepter is great choice if your opponents are mostly strong AP chars and you need something to be able to face them. This item won't only reduce the damage they do, but also decrease their own magic resistance to your spells!
Lich Bane is a highly controversial item, since it doesn't offer a lot to cover its prize and I mainly mean buying mana for Kata. The main reason why u can grab it is its passive, which increases your damage after any spell and combos great with your Shunp. However, since it costs a lot and is only partially useful, I would treat it only as an optional 6th item if you don't feel a need to buy anything else.
Other items - you will rarely find yourself under need to get them. If you play against a strong AP or AD centered enemies, you may consider getting more tanky items like Force of the Nature, Sunfire Cape, Thornmail etc to neglect their damage, or even late Warmog if you feel like you die to quickly before you can do any damage.

Why no Gunblade?

Hextech Gunblade used to be one of the most obvious items for malee AP champions like Kata or Akali. However, I say used because I feel the times of its greatness has ended with its nerf and now it's not really worth money it costs. In my build, Will of the Ancients is used as one of the primary items and there are really hard reasons for it. Basically - it offers you more for less money. If you already have bought Hextech Revolver and you are wondering what you should turn it into, well, let me tell you. You can either pay 900g for 25% Spell Vamp and 80 AP or pay 2425g for only 20% Spell Vamp and 70 AP, plus bonus AD and Life Steal - two things that aren't really useful on AP Kata and not really worth so much money. Yes, it's truth that you also get a nice passive, but with WotA you also share part of your AP and your 25% Spell Vamp with nearby allies, which is especially useful in TFs. Being able to grab it so fast makes your build more consistent and let's you concentrate more on your further items. That's why I pick WotA over Gunblade.

The best possible champions for Katarina's team would be the ones with best CCs, so they would support Kata's ultimate. As long as I have been playing, I weren't able to find any better champion to match Kata than Amumu. If you have any friend playing this champ - pick your Kata and check out the damage you'll make when you will combine your ulties. Just wait for mummy to do its stun, Shunp to him, do Death Losus and prepare for a large multi kill.
Other possibilities include strong stunners such as Morgana, Ashe, Lux, Sion, Sona, Taric etc. Just be in contact with them so you can charge just at the moment their stun is landed.

While playing Kata, you will be going mid lane 99% of time, so you have to be prepared to counter different AP carries you find there. Here is the list of most popular mid champions and the strategies you should use while playing against them.

Anivia

Just like in your case, Anivia is a champion that need her ultimate to show her wings. However, you must be aware of her nukes. Unlike you, she has range, so it may be a good idea to keep your distance and try only to harass with your blades. Beware of her stun and if she lands it while your low on health, better use Cleanse to escape her nuke. Also, it is really worth remembering about her passive - don't use your Death Lotus too early, while it may be a good way to destroy her egg by your own.

Annie

While Annie is not very dangerous without ulti, try not to get to close to her when she has stun. In other cases, you may try harassing her with both Bouncing Blade and Shunpo. Remember however, that she is extremely dangerous while she has her Tibbers and stun ready. Do not try to get close to her or eliminate her without help of your jungler.

Brand

Brand is not a great danger for Katarina, since his spells are basically easy to dodge with your early boots, which makes Brand's stun extremely hard to land. Watch for his ultimate though, as it may bounce through minions just like your blades. Other than that, feel free to harass poor Brand.

Fiddlesticks

Believe me... You don't stand a chance against this guy. His spell is even better than your blades and it bounces the same from the very 1st level, plus it also silences you, preventing you from spell vamping, while he can heal any time. Never mind, he can just stop your ulti in the middle of it. If you find him mid laning, beg for a change with your solo top.

Kennen

This little fellow can be really dangerous, especially since he can counter your ulti with his own one, so it really is better for him to do it, Cleanse away and then jump on him.

LeBlanc

Pretty similiar to Brand, but she has nice escape engine, so your ulti without stun does literally nothing here.

Lux / Morgana

You should be able to dodge their snares, and if you're not - use Cleanse when you feel they're going to land their combo on you. Just keep in mind that that two chars are extremely mana dependent, so just wait for them to unleash their charge and attack when they are out of resources. Shunpo, Cleanse and Flash will help you get out of most of troubles.

Malzahar

Another champion that unfortunately eats you like a bread. Malzahar is an extreme example of spammer, doing hundred spells every minute and every one of them having very high damage. Plus his ultimate. You can either play very defensively through entire game or have a change with solo top.

Sona

Wait... Sona on mid?! In this case that must be emachel, creator of this build... Never mind, just beg him for mercy.

Other options

As you probably realized, the strategy is simple. Apart from Fiddlesticks and Malzahar no real champion can dominate Kata (ok, maybe with the exception of rarely seen Vladimir), which means she can dominate them whenever they run out of mana and aren't prepared to face their opponent. Always harass with your blades, get last hits or more harass with your Shunpo and do your ulti when you have a chance to knock down your enemy.

Katarina in TFs does the same most of the time. Wait for your opponents to land their CCs or your allies to do theirs, Shunp in spot and do your ulti. Escape/chase, wait for cooldown and strike once more. Between TSs go to jungle or push minions to spell vamp remaining HP. Get to base every now and then when you gathered enough money for the item you need.

Thank you all for reading through all this guide! It really took me quite a lot of time to complete and much more time to gather all the knowledge and experience I need to complete it. Please feel free to check it out in game!
Besides, since this is just my 1st build ever, I would appreciate any possible feedback I can get from you! I'm waiting for your constructive comments and your feelings from reading this guide. Thank you and see you on the Fields of Justice!