Master Yi is a champion of many possibilities. He can be played either AD or AP. When played AD, he can be glass canon, tanky damage dealer or offtank. I think its best to build him as tanky damage dealer. You still have absolutely amazing damage.

eeper 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's 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.

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Pros

Great pusher/tower/farmer destroyer
Pretty easy to learn overall
CD resets/halves when you get a kill/assist(if Highlander active at that moment)
Great Escape

Cons

Vulnerable to disables like blind, stun,suppress...
Low HP poll until you get Frozen Mallet(or other HP item)
Not so great in team fights
Low magic resist and armor

JUNGLING RUNES

18 Armor Penetration || 13 Armor || 24 Magic Resistance at level 18 || 52 Health

9x Greater Mark of Desolation

When it comes to marks penetration runes are pretty much first tier. Armor penetration is extremely useful early game. It helps you take down jungle faster and makes your ganks easier. Combined with Armor Penetration Quints, your attacks ignore almost 20 armor, while basic AD carries start with about 15.

9x Greater Seal of Resilience

These are for early game survivability, both in jungle and lane. You start with very low amount of armor and if you go gank against ad carry which can crit you, you'll die before you even touch them. 13 Armor and start will greatly benefit your ganks and your jungling, since monsters do physical damage.

9x Greater Glyph of Shielding

These are also your defense resource. Many people prefer flat magic resistance Glyphs, which are also viable, but I take these because early game bursts aren't your threat. MR/level scales better in late game and helps you survive strong bursts from AP carries.

1x Greater Quintessence of Desolation

Like Armor Penetration Marks these are extremely useful early game. They stack nice with Marks and combined provide you 20 ArP, which means you deal almost true damage to squishies in early game when you gank.

2x Greater Quintessence of fortitude

Many people prefer 3x ArP Quints but I take a pair of these two because that extra 50 health early game is priceless. When you complete your jungle route you have more health which means you could go gank, while you would usually go back. I simply need to take flat health stats at start, otherwise it doesn't feel right.


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LANING RUNES

My basic runes setup is very similar to jungle ones. The only change here is that I take 3x Flat Health Quints instead of Armor Pen Quints. I decided to build that way for extra survivability early game.

15 Armor Penetration || 13 Armor || 24 Magic Resistance at level 18 || 78 Health

9x Greater Mark of Desolation

When it comes to marks penetration runes are pretty much first tier. Armor penetration is extremely useful early game. It helps you take down jungle faster and makes your ganks easier. Combined with Armor Penetration Quints, your attacks ignore almost 20 armor, while basic AD carries start with about 15.

9x Greater Seal of Resilience

These are for early game survivability, both in jungle and lane. You start with very low amount of armor and if you go gank against ad carry which can crit you, you'll die before you even touch them. 13 Armor and start will greatly benefit your ganks and your jungling, since monsters do physical damage.

9x Greater Glyph of Shielding

These are also your defense resource. Many people prefer flat magic resistance Glyphs, which are also viable, but I take these because early game bursts aren't your threat. MR/level scales better in late game and helps you survive strong bursts from AP carries.

3x Greater Quintessence of fortitude

Unlike jungle, when I'm playing lane I take three flat health quints. That makes me more early game tough and helps me be more offensive by dealing damage, yet surviving. I think that 26 health is way more advantage early game that 3 Armor Pen if you're laning.