Hi, i'm Raikoku, i play in EUW server, and this is my 1st guide on mobafire, so i'll try to be clear during the explaination of this choice. Katarina is my main mid laner, because she has a really high mobility, and she can do a lot of damage. It's also a good champ to start with. In this build, i'll talk about the item i use, why i use it, and the alternative build. also i'll talk about an advanced tactic that i call bermuda triangle ( i don't know if it has a proper name)

I usually uses an aggressive set of runes. the 9x mark of magic penatration make your pokes stronger, while the quints and the glyphs of ability power makes you start with 34 (more or less) ap at level 1, that's a pretty high value. Last but not the least, the seals of armor makes you more resistant against AD junglers during the laning phase and it also helps a bit during the teamfights.

The best summoner's spells on katarina are Ignite and Flash.
Ignite ensure you kills, while Flash, combined to your shunpo gives you an incredible mobility. you can chase the enemy even if they just flashed, and you can create a really big gap between you and your chaser. you also can take Heal instead of ignite, but i don't suggest it, because it makes you unable to get fed as Ignite does.

Before talking about the items, it's important to talk about farming, or creeping. or lasthitting, just to be clear. Katarina is not tanky, so you have to farm safe.
Level 1: you just gained your Q, Bouncing Blades). what you have to do now? when a creep (or a minion, it's the same thing) has low health, uses you q on it. your blade will bounce to the 5 nearest enemy targets. get closer to the low health creep and hit him. You can also last hit him with your basic attacks, in that case you have to hit him when the minion has less than 75/76 hp.
Level 2: you just gained your E( Shunpo) your E works like a basic attack reset, so if you are going to lose a creep, shunpo on it. you can also combo your q with your e (low life minion--> bouncing--->shunpo-->gold. this combo makes bouncing blade mark explode. crossed with shunpo, with this combo, if we don't considering the AP bonus, you do 135 damage.
level 3: you just gained my favorite Katarina skill. W( sinister steel) it's an AoE spell. It means that you damage the enemies around you. It also means that you can clean the lane pretty fast. the minions usually don't focus only 1 enemy creep, that makes you able to farm more or less 3/4 creeps at the same time using only your W. if u want to clean the lane even faster, combo your skills. q-e-w and go back. Remember: ALWAYS stand behind your minions.

i usually take this items as starting ones:

+3x Boots + x3 Health Potion

Why this pick? the boots makes you good at chasing the enemy, escaping and farming, while the pots counters a low level Ignite and they also make you able to stay in lane longer than the enemy.
1st back: I start buildin' up Haunting Guise if I get often ganked, and I can't handle it, i start with Ruby Crystal, else i go for Amplifying Tome. After it, at the 2nd back i complete Haunting Guise. I build up Liandry's Torment, passing by Amplifying Tome. Last early game item i take it's Sorcerer's Shoes

your early build will be like this

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This are the early game items i usually get. let's explain why.
Haunting Guise: It's passive gives you magic penetration, that means that you ignore 15 units of the magic resist of the enemy, it makes you stronger at poking and harassing.
Sorcerer's Shoes: harder, better, faster, stronger. An higher magic penetration makes you hard to get down, if you answer pokes back. Better magic penetation also means i can gank top or bot better, i do more damage, i can help my team. this shoes makes you faster in chasing enemies and escaping from them, quickly farming and quickly harrassing. Sorcerer's makes you hit stronger.
Liandry's Torment: Lot of people keep yelling me "N00B! Uninstall LoL, Liandry's is bad". that's probably the biggest lie ever made. Liandry gives u 50 AP, 300 HP, and you do percentual damage. 6% of damage over 5 sec. It means that you can harass really hard, and at the right time you can oneshot the enemy.
I ended my build for the early game, so i start buildin up my mid game stuff. 1st of all, Katarina it's not so much tanky, so i build up Rylai's Crystal Scepter step by step. I start from Giant's Belt, so i get lot of life and i can get enough farm/kills to get Blasting Wand, and after that, i buy Amplifying Tome to complete the item. next item is Needlessly Large Rod, to build up [[Rabadon's Deathcap].
you can also build Rabadon's Deathcap before Rylai's Crystal Scepter. It depends if you prefer to be tanky or to do damage.
your build at mid game will be more or less like this Sorcerer's Shoes Liandry's Torment Rabadon's Deathcap Rylai's Crystal Scepter
Little explaination why i take Rylai's instead of warmog. Rylai's gives me 500 hp, combined with the liandry i've 800hp bonus. It also give me 80 AP, while warmog gimme 0 AP bonus. Rylai's passive slows the enemy: 35% if it's only 1 enemy, 15% if we have multiple targets. It's passive stacks with Liandry's the percentual damage get's increased from 6% to 12% in 5 secs. It also make the enemy easier to hit, he's slowed, you are not.
Rabadon it's a must item on every AP mid laner, 120 AP+30% of actual AP. it means "kill kata before she launch his knifes!".
It's time to build up defensive stuff during the late game. the teamfights are every where, so a Zhonya's Hourglass may be usefull. it gives you 153 AP, scaled with rabadon's passive . it also gave you armor, you'll get less scars during a teamfight against AD champs. Zhonya's passive makes you untargetable, untouchable for 2.5 seconds, but it also make you unable to cast spells. during this time your team can save you, your Cooldown may be reset, so you'll be able to shunpo/flash outside this bad situation.

here i will put some other good items for katarina, i will explain when i take it and why.
these are the situational items that i find pretty great:

Void Staff Hextech Gunblade Abyssal Mask Will of the Ancients Warmog's Armor

Void Staff: the enemy has a big magic resist, i take it. you get 70 AP and you ignore 35% of enemy magic resist, it applies before the magic penetration. the magic penetration value goes from 8%(20 magic resist) to 30%(200 magic resist). it means that you do more damage because you spam better the enemy magic resistance.

Hextech Gunblade: if i need spell vamp, i'm in a mostly AD team and i want to buff up a bit my final, i buy this. it gives you 20% spell vamp, (so you are able to heal a bit yourself, and you also have a bigger sustain) 12% life steal (pretty useless on an AP champ) it also has a good active, 150 +40% of your ap damage and slow the enemy, and 60 sec of cool down.

Will of the Ancients: I prefer it if I need spell vamp and i'm in an AP team. it's aura increases also allies ap and gives them spell vamp. with this object gives you 80 AP, and 20 spell vamp. sustain and team utility, that's why i pick it instead of Hextech Gunblade

Warmog's Armor: i don't always take it, but it's a nice pick if you have an high AP and you need a tanky item. it can make you able to join the team fights earlier.

Abyssal Mask: it's a good early item against a counter, or against a strong AP enemy, because he gives you AP and Magic Resist. his aura makes you even stronger, it's like a bonus magic penetration imo.

I put here Deathfire Grasp, Sightstone and Enchantment: Alacrity because they are situational. If you want to do a really really high AP damage, and you also want an anti tank item, take Deathfire Grasp. Sightstone may seems a strange pick on katarina. as lee sin, it gives you a lot of mobility, it can really save you. i take it once, and it's pretty op. you can engage in a close fight even 3vs1, kill 1 of them and save your *** warding and shunpo behind a wall. it also grants vision to your team, that's a really really important thing during Teamfights and late game.
Enchantment: Alacrity it's another optional item, it's an alternative pick to the sightstone. faster movement speed means faster ganks, faster escaping, and faster chasing. crossed to your rylai's passive, in few seconds you are able to reach your target, kill him and go away before anyone reachs you.

You are playing as katarina, in mid lane. Even if you are playing aggressive, you have to play smart. my tipical fighting style is farming while poking. i'll explain it. The enemy champ is close to the farm, i cast my q on a creep close to the enemy champ, so he'll be hitted, then shunpo on that minion, and i use my sinister steel (w) to active the mark on the enemy doing higher damage, and i also get movement speed thanks to sinister steel, so i can go behind your minions before taking any damage. if we want to write it down as skill sequence, i do that to kill the enemy. q-->e-->w-->back until he have 300hp (every bar on the enemy champ equals to 100 hp) then the sequence become focused on the enemy champ: q-->e-->w-->r-->back. else, if i want to go more aggressive what i do is q-->e-->w-->r-->ignite, don't fear wasting ignite when you start your Death Lotus (R), you will get a 5 AP and 5 AD. It will secure you the kill if the enemy is not chopped down by your skill sequence.

let's start with the skill sequence and a rapid explaination of it.

Voracity: that's Katarina passive. it reduces your cooldown of 15 second for each kill or assist you make. this means that after getting a kill, or an assist your q, w and e will be ready again to chase an enemy. it has no cool down, this means that in a teamfight, if u help your team, obtaining kills or assist, your final will be refreshed soon, also you will be able to escape, if you are in a bad situation.
Bouncing Blades: this is the 1st skill i take. Katarina launches a knife on a enemy unit, the blade will bounce on the next 4 enemies and leave a mark on them, that last 4 seconds. every bounce reduces the damage of 10%. this skill is good for a safe farming and harrassing because it has a pretty high range(675). if you cast it on minions this spell make your farming easier, else you can use it to poke and harrass the enemy, and do extra damage once you detonate the mark. detonating the mark do a bonus damage on the marked enemy. you can detonate it with your skills and with your auto attack.
sinister steel: this is probably the main katarina's skill. it's an Area of Effect skill, which means you hit everything in it's range (375). when you hit an enemy champ, you receive a movement speed boost for 1 second. you can use this boost to go behind your minions after hitting him or to escape in bad situation. it's also a great farming skill. you go in the middle of your farm-->w--> lot of gold obtained. it's a great lane cleaner skill.
Shunpo: this ability gives you a lot of mobility. it makes you able to teleport behind wards, enemies, allies and creeps. you can use it for offensive, to complete your skill sequence, or as a gap closer. shunpo have 700 or range, so you can avoid kiting from any adc, (except for Tristana, her range is 703 thanks to her passive), or you can use it defensive, to teleport yourself behind a wall if there is a ward, or behind your ally.
it also reduces for 1.5 seconds the damage you take (15% damage reduction).
Death Lotus is Katarina final. this skill scales with her AP(200% of her ability power) and her AD(300% of her bonus attack damage). During death lotus, Katarina launches to the nearest 3 enemy champs up to 10 knifes during 2 seconds, and it also apply grevious wounds, which reduces they regeneration and healing by 50% for 3 seconds. this skill it's an offensive one, nothing to say. it's low cooldown makes it a really strong ulti, (50 seconds) and it gets refreshed with katarina's passive Voracity. the bad thing about this ulti, it's his range: 550 it's a little range, but you can overcome this little problem using your shunpo.
How to play a good katarina: you cannot.
Just kidding. the most important thing to know about katarina is that she needs to farm like hell during early phase. At level 1, you have no so much power to farm quickly and safe, so use your Bouncing Blades on low life minions and your basic attack to do bonus damage and farm quickly. focus low life minions 1st. remember to play safe. you can poke the enemy, but it's not necessary. better wait level 2 to real pokes. at level 2, gaining Shunpo you can easily take down minions with your q-->e combo, you also have to start poking your enemy with q-->e and running behind your minion, and try to outfarm him.
at level 3, your ability to farm and poke grows really up. sinister steel, combined with q-->e combo clean the lane. when i use sinister steel i take down from 2 to 6 minions, in early game. you poke now it's really strong. use the q->e->w combo on enemy to poke him, unless he is under tower, in that case, only use your q from long distance. it's not important if he's the 1st target of your q. if you are getting ganked, take 2 wards and put them in the brushes. they will save you from ganks and also make you able to jump away from CC. at level 4,5 and 6 keep on farming and keep on outfarming the enemy. call your jungler and try to get kills. when your enemy is far from the turret ( at the middle of the mid lane), and have less than 600/700 hp, he becames your prey. use the combo q-->e-->w-->r to give him tons of damage. if this don't kill him, use your ignite. at level 7/8, if you farm well, you have a level advantage against the other lane. it's time to gank. Remember: Communication and cooperation are the main key to win. Tell that you are going to gank, and smart ping it out. hide in the brush, and wait the enemy push. at the right time, (the right time is when he is far from his turret and you stops his path to run back) start the gank using your q, and asap your e. faster you do this, better will be the gank. the difference between a katarina and a good katarina is the laning phase and her ability to avoid skill shots and strong CC like silence, charm or taunt. A good katarina doesn't need her final to kill the enemy. you can make a lot of combo, all of them are dangerous. the best one is q-->e-->w, but another one to do is q-->w-->e, here you use your Shunpo to get to a safer place. I always suggest an aggressive game, so i use mostly q-->e-->w combo, also because the enemy run under turret when you start this combo. Do we want to take 150 free damage and risk to not obtain the kill? no, we don't. you can steal enemy red if you feel confident enough to go alone in the enemy jungle. or ask to the jungler if you can take it.

one of the most important thing that you have to learn when you play mid, is to gank. you want to win fast? gank bot and top. you won't lose time going top, same going bot. you have to keep an eye on the other lanes. if you gank, you can get fed and win your lane. talk with your team, ask them if they need a gank, talk with the jungler to do a double successfully gank. during the teamfights, your role is to do damage. you are not rambo, you are not tanky. (with this built you will have more or less 2800hp, not so much) you must never ever engage. let the jungler or the top that role. you have to dive in the teamfight when the enemy team has medium/low life. If you don't get kills during a teamfight, but you get assist don't worry. Katarina's passive, Voracity reduces your cooldown of 15 sec for assist or kill.

Pros high mobility easy to learn tons of damage strong harass low Cooldown great passive great ganks

Cons: no CC easily coutered by AD champs squishy Item dependant she gives her best only at mid needs to farm

playing in mid lane, there is a particular tactic that you can use. I call it the bermuda triangle. near the mid, at your left there are 3 brushes. you have to bait the enemy in this triangle of brushes. after doing that, you have to run in the 3 brushes, making a triangle, while you spam your w. you will escape from the enemy, and you will do a lot of damage. when he will try to escape, cause he will have low life, shunpo and kill him. this tactic have it's limits. you cannot do that if there is a numerical disadvantage.

I was thinking on opening a twitch channel, where i'll show how i play katarina, and listen to your suggestion to be a better gamer. i also glad if anyone of you wants to play with/against me, so if you want to add me to your friend list feel free to do that. if you have any question, PM me.
and remember: play aggressive, be smart and Cooperate.

"never Question my loyalty. You will never know what I endure for it"- Katarina