Keeper of the ancient art of Wuju, Master Yi is a mystical warrior from the isle of Ionia. He descended from one of the few tribes dedicated to the preservation of Wuju, a martial art founded with the principles of absolute spiritual awareness of one's self and one's enemy. When the armies of Noxus invaded Ionia, Master Yi joined in the Ionian defense to fight the craven enemy, bringing his village honor through his unparalleled mastery of the blade. The ensuing battles ravaged the land, but no conflict inflicted as much horror as the day the armies of Noxus closed in on Yi's village. Left undefended, the village became the unfortunate test subject of the latest invention by an up-and-coming chemist, now known as Singed. The remains of the village that Master Yi returned to were so horrific that it remains a festering scar on Ionian history. To this day, no Ionian will ever publicly speak of what occurred.

After the Noxian invasion was contained, Master Yi fell into a deep isolation. For years, he trained morning to night, speaking to no one and refusing offers of consolation by fellow Ionians. The only thing stopping him from madly charging into Noxus battle lines headfirst was his pledge to keep the Wuju style alive. Even though he seethed with impatience, his rage became a tool for sharpening his practice of Wuju, developing stronger and deadlier skills for his eventual vengeance. The only thing that roused him was news of the League's formation, and Noxus' subsequent enrollment in the League. Master Yi now brings his mastery of the blade to the League of Legends with unflinching purpose: to avenge his kinfolk and end the predations of the Noxus once and for all.

''The art of Wuju lives on in Master Yi, and is plunged through the hearts of his foes.'' -- Soraka, the Starchild

PROS

Carry
AD Nuker
Great Chaser
Great Back Door-er
Long Laner (Meditate)
Great Escape Skills
[Late Game] Can nuke Squishies in 2-4 Hits (1-3seconds)

Cons

Squishy
Focused during fights

I chose the 21-0-9 Mastery because Master Yi is a AD Carry and needs the highest damage outcome possible. Increasing Critical Strike Damage by 10% and total Damage by 4% as well as Armour Penetration while still having decent mana regeneration for long laning.

Armour Penetration Marks to Decrease damage deduction.
Critical Strike Damage Seals for Increased critical strike damage.
CoolDown Reduction Glyphs to Reduce Cooldown timer.
Armour Penetration Quintessences to Decrease damage deduction.

First addbecause it's the most useful at level 1, using it to farm minions and chase champions.

Then I recommend maxingfirst whenever a skill point is possible. This skill is extremely helpful at low levels, It helps you lane WAY longer than normal and it will allow you to heal yourself more than 2 champions could damage you.
<- Not much to say. Add at level 6, 11 and 16. Great Chasing as Escaping skill.

Skill importance:

=>=>=>

Recommended Summoner Spells:
Ghost
Exhaust

Alternative Summoner Spells:
Flash (Ghost Alternative)
Cleanse (Anti-CC)
Ignite (Ensuring a Kill/Decrease Healing)
Teleport (Great for Backdooring)

Start off with aBrawler's Gloves, aHealth Potion and aMana Potion.

Quickyly Build theBrawler's Gloves into aAvarice Blade, then buyBerserker Greeves.

TheExecutioner's Calling is optional, but recommended. Its like a ignite that has a 20 Second CoolDown.

Try to rush aInfinity Edge ASAP, the item sequence to build it doesn't matter, just need to rush it to start AD nuking everyone.

When you have a Infinity Edge aPhantom Dancer is a MUST have.
After you get your first Phantom Dancer, Rush another one.

Then sell yourAvarice Blade for aBloodthirster Once you have enough to completely Build it.

Selling yourExecutioner's Calling for anotherBloodthirster Once you have enough to completely Build it.