Master Yi, The Wuju Bladesman is a melee champion. Before the 3.10 Patch he was commonly played in two different styles, either as a melee carry with a focus on building attack speed, or as an AP carry, as his Alpha Strike and Meditate scaled very well of AP. However since the rework it's not efficent to build AP on him anymore. His kit gives him a couple of powerful steroids and a channeled selfheal with additional defstats. With his ultimate on, he is immune to slows, and also gets cooldown resets upon his other abilities when achieving kills or assists. His limitations are his total lack of any form of crowd control and the fact, that he has to get into melee range for his autoattacks, while being relatively squishy. His selfheal can be interrupted by hard crowd contral such as stuns, knockups and knockbacks.

9 Greater Mark of Armor Penetration
9 Greater Seal of Armor
9 Greater Glyph of Scaling Magic Resist
3 Greater Quintessence of Attack Damage

18-12-0

Double Strike
(Innate) Every 4th consecutive basic attack, Master Yi strikes twice. The second attack uses an attack damage modifier of 50%, but applies on-hit effects fully and can critically strike.
Double Strike's counter resets if Master Yi has not dealt or received damage in the last 4 seconds.

Alpha Strike
(Active) Master Yi becomes untargetable and leaps to strike up to 4 enemies, dealing physical damage to each and dealing additional damage to minions and monsters.
Each bounce will target the nearest enemy that has not yet been damaged. Master Yi re-appears at the initial target's location after the effect ends. Alpha Strike can critically strike, dealing additional physical damage equal to 60% AD. Basic attacks lower the cooldown of Alpha Strike by 1 second.
Range to First Target: 600
Cooldown Reduction on Auto-Attack: 1 second
Cost: 70 / 80 / 90 / 100 / 110 mana
Cooldown: 18 / 17 / 16 / 15 / 14 seconds
Physical Damage: 25 / 60 / 95 / 130 / 165
(+1.0 of Attack Damage)
Bonus Damage to Minions: 75 / 100 / 125 / 150 / 175

Meditate
(Active) Master Yi channels for up to 4 seconds, restoring health each second. This healing is increased by 1% for every 1% of his missing health. While channeling, Master Yi reduces incoming damage. This damage reduction is halved against turrets.
Cost: 50 mana
Damage Reduction: 50 / 55 / 60 / 65 / 70%
Heal: 30 / 50 / 70 / 90 / 110
(+0.3 per Ability Power)) per second
Maximum healing per second 60 / 100 / 140 / 180 / 220 (+0.6 per Ability Power)
Base total heal 120 / 200 / 280 / 360 / 440 (+1.2 per Ability Power)

Wuju Style
(Passive) Master Yi's attack damage is increased by a percentage.
(Active) Master Yi deals true damage on hit for 5 seconds. Wuju Style's passive bonus is lost while the skill is on cooldown.
Cost: 0 mana
Cooldown: 18 / 17 / 16 / 15 / 14 seconds
Bonus Attack Damage: 10%
True Damage: 10 / 15 / 20 / 25 / 30 (+0.1 / 0.125 / 0.15 / 0.175 / 0.2 of total Attack Damage)

Highlanderg
(Passive)Champion kills and assist reduce the cooldowns of Master Yi's other abilities by 70% of their remaining cooldown.
(Active) Grants Master Yi Attack Speed and Movement Speed for 10 seconds. If Master Yi scores a kill or assist while Highlander is active, Highlander's duration is extended by 4 seconds.
Cost: 100 mana
Cooldown: 75 seconds
Attack Speed: 30 / 55 / 80%
Movement Speed: 25 / 35 / 45%
Duration: 10 seconds + 4 seconds every kill/assist

Through the ancient martial art of Wuju, Master Yi has tempered his body and sharpened his mind until thought and action have become one. Though he chooses to enter into violence as a last resort, the grace and speed with which he wields his blade ensures resolution is always swift. As the last living practitioner of Wuju, Master Yi has devoted his life to finding able pupils to carry on the legacy of his lost people.
Even before Yi mastered Wuju, he was considered one of the most skilled practitioners of the mystical martial art. He would soon prove his mastery when word of a massive Noxian invasion reached his remote village. Yi swept across the battlefields of Ionia, turning back the tide of Noxus's vast infantry with swift and deadly strikes, much to the embarrassment of Noxian High Command. Recognizing the threat the Wuju disciples posed to their invasion, the Noxians chose to unleash a nightmarish chemical attack on the home of the deadly art. Those who somehow survived the poisonous concoction had their minds twisted beyond repair. Yi's home was left in ruin.
At the war's conclusion, Yi returned to the grotesque remains of his village. There he became the attack's final casualty. Slain in spirit, if not in body, Yi clung to the only feeling left within his heart: vengeance. Driven only by his desire to punish those who'd destroyed his home, Yi spent years training in seclusion. He became a deadlier swordsman than he had ever been, but true mastery of Wuju still eluded him.
At the height of Yi's frustration, a monkey of unusually noble bearing interrupted his training. Standing as straight and tall as a man, the monkey watched and mimicked Yi's movements. Yi shooed the monkey away, but the agile creature took great amusement in turning Yi's own techniques against him. Gradually, Yi felt his anger subside as he sparred with the playful animal, and when the burden of his hatred had fully lifted, he found he had caught the monkey by his tail. Yi then understood that he would never master Wuju so long as he pursued it for vengeance, and as he let go of the monkey, he also released his desire to shed his enemy's blood.
Yi thanked the monkey for showing him what he'd been blind to, and was surprised when the creature actually replied. He wished to learn Yi's art of fighting. It was an odd request, but through it Yi saw his new path: the way to honor the memory of his lost people was to pass their teachings on to a new generation.

Master Yi is ultra viable in ranked for it's Crazy damage output, high mobility.
Master is very easy to play and learn.
Master Yi is an amazing elo boost champion, especially for Bronze/Silver league players.

Master Yi's strengths:
Has completely ridiculous damage output
High mobility
Can snowball out of control easily (Can carry games hard)
One of the most efficient split pushers
Farms easily and quickly
Pretty much the best champion to start playing the game with (easy to learn, extremely powerful at low levels/elo)

Master Yi's Weaknesses:
No pressure early game because he needs to spend most of his time farming instead of ganking
Brings no utility at all for teamfights (No CC)
Needs to take a lot of farm to be effective late game
Will be the number one priority target for the enemy team most of the time