This is a guide for Master Yi, one of the lords of Twisted Treeline. He is played, by me at least, as "the janitor", going in and cleaning up the fights. Some of you might notice that I have made another Yi guide, also for Twisted Treeline, but I realized that that build was **** compared to my current one. I have chosen to leave that other build still up (Master Yi: The 3v3 Shredder) because it still seems to work sometimes. I feel that this build/method of playing Yi on Twisted Treeline is MUCH better though. Without further ado, I present Master Yi, King of the Twisted Yiline.
Note: I wanted to make this guide as detailed as possible, so forgive me if it is really long.

Master Yi is a wonderful champion. He is a melee DPS, capable of backdooring, obliterating enemies, and soloing Ebonwaw at low levels. I probably left quite a few roles out there, but I'm sure anybody reading this has either played Yi or seen him played.
Earier, I mentioned the "Lords of the Twisted Treeline". I think that the best champions on Twisted Treeline, the ones that can almost guarantee a win if played correctly, are Master Yi, Xin Zhao, Tryndamere, and Jax. I will explain why later.

On Master Yi, I take mostly defense. This is partially due to the fact that I have only 2 rune pages, annoyingly, and also, Master Yi is a huge squishbag. My runes are:
Greater Mark of Desolation. I looooove armor penetration because it boosts your damage output by a lot. This is my only attack rune.
Greater Seal of Armor. Armor on these squishbag champions is always nice. Since I don't usually get a lot of defensive items, these seals help my survivability.
Greater Glyph of Magic Resist. I get these for the same reason as the seals: Yi's squishiness. Magi rip him up, so the less they do, the better, right?
Greater Quintessence of Health. Flat health Quints for early game. No brainer. Unless you want more ArP. That's also viable. Your choice on these.

On Master Yi, I take 21/0/9. The crit chance and damage help, so does ArP and all the other stuff I grab. On the Utility tree, the improved regen for mana and health are always helpful, along with a nice level advantage helper, and increased duration on buffs.

My summoner spells that I take on Master Yi are Exhaust and Ignite. Since Yi has NO CC, and I can't always have Red Buff, Exhaust is wonderful. It also is helpful if facing the other Lords, by denying their damage. Ignite helps finish off kills, stop healers, and gives some damage buff to Alpha Strike and healing buff to Meditate when it's on cooldown. I know Cleanse is recommended by most people on him, but Mercury Treads cover for that problem, and flash is good sometimes, but doesn't always do great. You find yourself relying on flash to get away, thinking "Oh, I can overextend, if I get ganked, I'll flash over a wall". Then you get ganked. And you miss the wall. You are now dead. I got a Triple Kill first blood a few hours ago, and these spells made it all possible.

Wriggle's Lantern is a necessity, considering you will be spending much of your time in the jungle. It speeds up your jungling by a lot. Later in the game, when you have no real need to farm a lot, you can trade it for Zeke's Harbinger, Bloodthirster, or Executioner's Calling.
Mercury's Treads are probably the best boots for Master Yi. He is a VERY squishy champion, and since I don't take Cleanse, and hopefully won't need Quickilver Sash, the Tenacity is wonderful. Any added survivability in the form of Magic Resistance and Tenacity is always good, because as my friend put it, "Master Yi is a global taunt".
Youmuu's Ghostblade is pretty much great on every melee DPS. AD, CdR, ArP, Crit, and an active that dominates in team fights and makes backdooring exceptionally easy all mashed together makes for a perfect item.
Phantom Dancer boosts your attack speed by a large amount, increases the effectiveness of the armor penetration from Youmuu's with lots of Crit, and helps you run around even faster. With Youmuu's Active, Highlander, and Crest of Flowing Water, you can chase or escape anything.
Infinity Edge is a huge buff to your damage output, with straight AD and crit chance and damage. If you EVER get to this item on Twisted Treeline... then you need to backdoor more. Anyway, great item, if rather expensive.
Black Cleaver gives a nice damage boost, with a bit of Attack Speed. The Unique Passive makes you do even MORE damage, somehow, by dropping the enemy's armor by 45 if you can land 3 hits on him.
Bloodthirster, Zeke's Harbinger, Executioner's Calling are late game items which are probably not going to be touched. You can pick and choose which one you want to replace your Wriggle's Lantern. Bloodthirster gives a ton of damage, and the most lifesteal. Zeke's Harbinger gives a lot of attack speed, almost as much lifesteal, and aura, and reduces armor of nearby enemies by 20. Great synergy with Black Cleaver. Executioner's Calling gives a bit of crit chance, a nice bit of lifesteal, and a wonderful active for shutting down healers.
Yes, I know that these items are exceptionally expensive, and you probably won't get to all of them. The reason being, of course, that if you have these items, the game is already over. The expense is not a problem because of how you will be in the jungle a lot, getting tons of gold, and farming whenever possible.

There are a lot of items that have great synergy with Master Yi, but, alas, only 6 are allowed the honor of being used by him. Some items that didn't quite make the cut are shown here:
OFFENSIVE:
Cloak and Dagger When facing teams heavy on CC (like Blitzcrank and Amumu, etc), this item helps solve that while giving AS and Crit.
Guinsoo's Rageblade Not the greatest item unless you are playing hybrid Yi, but still pretty nice with its passive (stacks of AS and AP)
Malady and Nashor's Tooth are both like Guinsoo's in that they are good for hybrid Yi, and Malady applies an onhit effect.
Stinger without going all the way to Nashor's is pretty good, AS and CDR, and fairly cheap.
Sword of the Divine Straight attack speed with a small onhit effect. VERY good against Jax.
Madred's Bloodrazor If going up against teams with huge amounts of HP, this item can help obliterate them.
Wit's End Give you magic resist stacks, which is useful vs. magic heavy teams.
Frozen Mallet A VERY nice permaslow, and some health and damage. Fun to get if you're playing on a smurf.
Hextech Gunblade Good on hybrid Yi, and somewhat good on regular Yi.
Last Whisper Some AD and some precious ArP.
Manamune I don't usually have huge mana problems, but if you feel that you do, get mana seals (tier 2 if you want, there's not a huge difference)
Sword of the Occult If you think you will get REALLY fed, grab this. If not, it's a waste of money.
Tiamat Splash damage is cool.... but not extremely useful. Only in customs/ smurf games.
Trinity Force A really good item. A slow that is almost sure to proc considering your nice AS, and lots of bonus damage every time after you cast a spell. Also, health, crit chance and AS are good too.
DEFENSE:
Guardian Angel If you are getting mauled and want a second chance, here you go.
Banshee's Veil Blocks spells, give health and mana, and MR. Good item vs big time casters.
Quicksilver Sash Gives MR and a free clease ever 1.5 minutes (as of patch v1.0.0.120)
Warmog's Armor If you just want some solid health.
Thornmail Only if you ABSOLUTELY need it. Against, say, Tryndamere.

Master Yi is a very odd champion. He can be used effectively by new people and experienced people. When people see him, they think "Oh, it's Master Yi, this person is a n00b". Sure, a lot of Master Yi players are really bad, but this stereotype is a good thing. If you are underestimated, then you can win. Now, I'll start to detail how to play Master Yi.

At the beginning of the game, grab Boots and 3 Health Potions. Beforehand, in the champion selection screen, tell your team that you will be solo top, while hitting the jungle a lot. Your team will accept this, or they are stupid. If they refuse to let you go top, wait until you have your Wriggle's, then jungle more full time than you otherwise would. You will soon be equal leveled with your top. This guide, however, details what to do if you go TOP.
As you are running to the bush, ask your team, or make a decision yourself, if you are going to charge into the bush. The gank can go either way. I have gotten Triple Kill first bloods, but I also die in the exchange somewhat frequently. My advice is if the enemy has any early game high damage moves or slows, DON'T GANK. Just go straight to your lane. Make sure to not run through the middle of the jungle, as their team could also be there.
Use Alpha Strike to kill minions and harass at the same time. For example, if the enemy wave has only three minions in it, Alpha Strike is GUARANTEED to hit the enemy if he is in range. Don't actually go to him unless you are willing to commit to killing him. Try to get 1600 Gold before recalling. That will buy you your Wriggle's. To do some heavy harassment to enemies, or to kill them, charge up a Double Strike, then run into a bush. Pop Wuju Style, then Alpha Strike out of the bush TO THE ENEMY. I know that contradicts what I just said, but I just wanted to make sure you understood that. Double Strike, with Alpha Stike, will do quite a bit of damage. If you want to kill him, Exhaust and Ignite him while whacking his brains out. He should die.
Finish your Mercury's Treads and begin building Youmuu's, starting with The Brutalizer.
I typically don't gank at early levels unless a very easy kill appears. I try to farm and get a nice level advantage before ganking bottom.

By now, you probably have finished your Mercury's Treads and are starting to work on Youmuu's Ghostblade. I will now detail how exactly you should go about jungling.
If in the top left and right camps, there are a Wolf and two lesser wolves, use Alpha Strike. Basic attack the remaining wolves.
For Wraith Camp, lead with Alpha. If you get lucky, you will kill all of the wraiths instantly. If not, just use your basic attacks.
For the assorted monsters camp, use Alpha strike, then just basic attack whichever ones you want first.
For Grez, the Lizard Lord, use Wuju Style while hitting it. If you want someone to help, that's fine, as long as they don't use a DoT.
For the Ghast or Rabid Wolf, lead with Alpha Strike. Then move back to the brush so you can keep an eye on your surroundings. Focus the biggest one, then kill the rest. Wuju style will not be necessary at later levels, but early, you might need to use it. MAKE SURE YOU NEVER DROP BELOW 120 MANA!!! I cannot stress this enough. Highlander costs that, and you might need it to get away.
Ebonwaw is a formidable opponent. Wait until you have at least Level 9 and Wriggle's before attempting to solo it. Lead with Alpha, then pop Wuju while it is at about two-thirds health. That way the period of no wuju at all is after you are done.

Whenever you can see at least 2 enemies on the screen, it is usually fine to hit the jungle and get buffs. The only exception is if the missing person is one of the other Lords. Or Garen. Garen's bad news due to his huge smite-you-from-the-face-of-the-earth move.
Once you get Red Buff, and hopefully Black Buff, gank. Just dash in, exhaust someone and beat their brains out. Use ignite if you want. Up to you.

By now, you should be able to 1v1 any champion, excluding, like always, your fellow Lords (Tryndamere, Xin Zhao, Jax, in case you forgot). However, if the game is still going on, the other team is obviously competent. In that case you will need to backdoor. I know you're thinking "Backdooring? On Twisted Treeline? Is he nuts?". Quite the contrary. Say the enemy is having a poke battle with your other teammates bottom. A small minion wave is against the enemy top turret. Run up to it, use E R 1 (if 1 is Youmuu's) to destroy it in less than five seconds. Then, a few of them will probably come to intercept you. Luckily, TT is a very easy escape map. You can escape into the top jungle where they can't find you. Then they will give up. FREE JUNGLE TIME! Remember the Yi motto: farm farm farm farm farm etc.
In teamfights, let your tanky people or the enemy start it. When everybody is a bit hurt, dash in, pop all of your abilities (except meditate, obviously) and focus their highest damage dealer. Continue on to ace them. Then push. Or get dragon. It's really up to you, though pushing yields better results, generally.

Double Strike Is his passive. Master Yi strikes twice every 7th attack.
This passive is what sets apart bad Yis from good Yis. A bad Yi will think, "This passive sucks. It doesn't do anything." Yeah, right. It can cause an enemy to go from thinking, "We're both low health. I'm gonna beat him!" to "Holy ****, how did that do so much damage!?!?!?!" It gives some nice burst damage to him.

Alpha Strike This is his Q move. Master Yi leaps across the battlefield with blinding speed, dealing magic damage to multiple units in his path with a chance to deal bonus magic damage to minions.
Master Yi leaps across the battlefield striking up to 4 enemies, dealing 100/150/200/250/300 (+1 per ability power) magic damage to each enemy with a 20/30/40/50/60% chance to deal 400 bonus magic damage to minions.
Cooldown - 18/16/14/12/10 seconds
Cost - 80/90/100/110/120 Mana
Range - 600
This is a great distance closer and harassment move. In some cases, you should not use it right away. For example: Tryndamere charges into a 1v3, and you and your team obliterate him. He uses Undying Rage, and starts running. Guess when he will spin away, then use Alpha on him. You will follow him and then 1 shot him. No problemo. Great for farming monsters, too.

Meditate Yi's W move. Master Yi rejuvenates his body by focus of mind, restoring health and increasing his armor and magic resistance for a short time.
Master Yi channels, restoring 140/280/420/560/700 (+1.65 per ability power) health over 5 seconds. While channeling, Master Yi gains 100/150/200/250/300 armor and magic resistance.
Cooldown - 35 seconds
Cost - 70/85/100/115/130 Mana
No range: only works on self.
A greatly underestimated move. AP Yi puts more of an emphasis on it than AD, but it is valuable on any Master Yi build. It can help you tank people, due to its massive heal and Armor and MR buff. It can heal you after a fight, so you can continue pushing or farming or whatever it is you want to do. Also, it looks really funny when he does this move while watching Nidalee's /dance.

Wuju Style Yi's E move. Master Yi becomes skilled in the art of Wuju, passively increasing the power of his physical attacks. Activating Wuju Style doubles the passive bonus damage for a short time, but the passive bonus is temporarily removed after the effect wears off.
Passive: Increases physical damage by 15/20/25/30/35.

Active: Master Yi can activate this ability to double its effectiveness for 10 seconds, but this removes the passive bonus until Wuju Style becomes available again.
Cooldown - 25 seconds
Cost - 40 Mana
No Range: Only for self
BUWHAHAHAHAHAHAHAHAHA! Epic damage steroid! I typically leave its passive on while farming, and turn it on during fights. I usually level this about equally with Alpha Strike. With the active on, you get a whopping 70 AD.

Highlander Yi's Ultimate and R move. Master Yi moves with unparalleled agility, temporarily increasing Master Yi's movement and attack speeds as well as making him immune to all slowing effects. Additionally, killing a Champion refreshes all of Master Yi's cooldowns.
Increases Master Yi's movement speed by 40% and Attack Speed by 40/60/80%, and he becomes immune to all slowing effects for 6/9/12 seconds. Additionally, killing a champion refreshes all of Master Yi's cooldowns, and scoring an assist reduce the cooldowns by half.
Cooldown - 75 seconds
Cost - 120/120/120 Mana
Hoo boy. This is a great Ult for many reasons. First of all, you can escape very well with it. You get much faster and can't be slowed. Second, you can chase down kills. Third, it increases greatly how fast you attack. Finally, every time you get a kill with Highlander on, it recharges all of your cooldowns (except for summoner spells, of course). Use it usually for kills, escapes, or backdooring if you are losing and need to kill the turrets quickly. If you get a kill with this on, Wuju Style's Active and Passive will overlap, giving 105 bonus AD for a while. LOLZOMG!

As you can see by looking at the top, I level my abilities like this:
AS
WS
AS
M
WS
H
AS
WS
AS
WS
H
AS
WS
M
M
H
M
M
1 point in Meditate early pays off a lot, but leveling it first or second seems kind of stupid.....
I don't level Alpha first because it costs too much, and I'm playing AD Yi, not AP. I also don't level Wuju all the way first because then my dash move does no damage and is on an insanely long cooldown. I think a healthy mix of leveling the two together is perfect.

I have mentioned the other Lords, or Kings, of the Twisted Treeline quite a few times. I feel that it is only right to say how to counter these other three. Once again, they are Tryndamere, Xin Zhao, and Jax.

Tryndamere is very strong, but is somewhat different after his rework. Unfortunately, he is still almost as strong as he was before, at least on Twisted Treeline. Luckily, he is fairly simple to counter. Your options are A) shut down his farming. Kill the jungle before he does, don't let him farm. Easier said than done, because he is a very efficient jungler at higher levels. Option B) Grab a Thornmail. Very simple, and it will work very well in countering him. Unfortunately, Mocking Shout can also shut down your damage, which is a major hindrance, obviously.
To actually KILL him, you need to play smart. Do NOT engage him 1 on 1 if he has his ultimate, unless he really sucks and you are fed. You can, however, fight him until you see/hear him use Undying Rage, then start running. He will probably chase you, knowing how most Tryndameres are cocky idiots. Then, you flip around, Alpha Strike him, then ignite. Make sure that you always wait until he is partially into his ult before igniting. It pretty much gives a guaranteed kill on him.

Jax is also very strong on Twisted Treeline. He can be built either AD or AP, or a combination. He is a very strong burst champion, and his dodge stuff absolutely WRECKS Master Yi. Unless you can obliterate him early, making him ragequit, you will almost definitely need a Sword of the Occult. Do not engage him in a minion wave, because then he will constantly stun your brains out. To counter him: Again, SotD. Thornmail, if he is playing AD. If he is playing the build that is increasing in popularity of multipe Hextech Gunblades, an Executioner's Calling might be a good investment to shut down his healing.
Pretty much, if you want to beat Jax, don't go 1v1. His stunning is just too crazy for that.

Xin Zhao doesn't seem to be as popular as the other Kings on Twisted Treeline, but I think he is equally viable. His knockup is what makes it almost impossible for Master Yi to 1v1 him. Unless he is low health, try to avoid getting in a duel with him. In a team fight, you can focus him, because he will not be able to focus one person in particular during the fight, as you will be flying around like a little birdie of death.
To counter him: Thornmail is always good vs. AD carries. Other than that..... just don't get caught alone with him.

After seeing these reviews of these champions, you might be thinking "Why would I play Master Yi over these other guys, if they can beat him in a 1v1?" Well, you see, the game, believe it or not, is NOT about how many champions you kill. Nor is it minions. It is whomever kills the enemy nexus first. And a champion that is very good at 1v1's, if maybe not the very best, who can also kill a turret in four seconds, is an ideal champion for Twisted Treeline.

Master Yi is one of the best farmers in Twisted Treeline. When you are in lane, you can use Alpha Strike to get minion kills. Also, make sure you get as many last hits as possible. Try doing a custom game to get the hang of his attack animations so you can be a very effective last hitter. Personally, I don't use Wuju Style's active while farming. It seems to just drain mana and give a damage DEbuff while still farming. Not fun. Beyond that, all the advice I can give you is ALWAYS BE FARMING. Low health, but you know where the enemy is? JUNGLE! Full health? Lane! You get the picture....

I see lots of mistakes while facing other Master Yi's playing. One major one is initiating with Alpha Strike at low levels. It has such a long recharge when it is low level that you can't use it again for a while, so the enemy can escape easily.
Another mistake is using Highlander to get TO battles. No. You use it when you are IN battles so you can kill people faster. That way it recharges instantly.
So here is how to do things right:
At low levels:
Run in. Pop Wuju Style. Wreck them. Use Alpha Strike to finish them.
Or: Run in. Pop Wuju Style. Get wrecked. Run away, maybe using minions to leap off of with Alpha.
At mid levels:
Run in. Pop Wuju Style. Wreck them. Pop Highlander. Use Alpha Strike to finish them off.
Or: Run in. Pop Wuju Style. Get wrecked. Pop highlander. Run, run, as fast as you can, you can't catch me, I'm the Gingerbread Man! (with a sword)
At High Levels:
Alpha Strike in. Pop Wuju Style and Highlander. Use Alpha Strike if you have to, and they are dead.
Or: Sorry. There are no alternatives. You will not die. Or need to run away. You are fed Master Yi, and nothing can kill you.

Well, I hope you liked my guide for Master Yi: King of the Twisted Yiline. (See what I did there? YIline? hahahaha, I'm so clever). If you saw my previous build, Master Yi: The 3v3 Shredder, and didn't like it, I hope you like this. I'm not saying that this guide will GUARANTEE success, but..... Unless you are facing another King or have a leaver/feeder, you will probably win. Feel free to leave any suggestions, comments or concerns in the comment section, and don't forget to vote! If you downvote, please leave an explanation, I'm always looking to improve.
If you want to friend me in game, my name is Nerdybeast.