Hello My name is Divorce, I'm a very dedicated Rek'sai Main That has been playing since season 1.

I'm a Firm believer that Rek'sai can be played any lane with some sucess. Rek'Sai works nice as a jungle. But junlers normaly have less resources than laners, which makes them play from behind most of the game.

But Rek'sai is not useful from behind, that is why i choose to play her mid because she has access to a lot more thing, including pretty sick roams to top and to bot to get your team ahead.
You pretty much want to shove and roam all game.

Rek'sai also works very well as a toplaner to perma shove and take down tower very fast. She can also be a good support with very agressive adcs.
I have also tried Rek'sai adc with blitzcrank and it works wonders.

But i digress this guide is for Rek'sai mid only.

Rek'sai Mid is a wild card, that can be very efective because most people do not even know what her ult does. And she has a crazy amount of burst, she also shoves very well and being intended for jungle she is an amazing ganker with very unpredictable gank paths for roams.

Before anything else is very important to discuss the Rek'sai mindset, you are gonna be a very agressive second jung you need to ward often and get good deep wards on the enemy jung. Rek'sai is very strong especially early game, she has an insane level 3 spike. You should play around her power spikes. I personally always all in level 3 and get a kill.

Idealy you will kill the enemy, shove and then gank other lanes or invade the enemy jung, steal buffs, kill the jung and then return to lane.
If you are eficient you will get lots of gold really early.
In a Rek'sai mid game I normally get my mithic at 7 or 8 min into the game that is how good this strategy is, there is almost no downsides to this.

Just always remember to not steal every thing from your team mates because you might lose since Rek'sai falls off late game. You need to spread your advantages to your team.

I think the notes speak for themselfs, Hails is by far the best dps keystone for rek'sai, cheap shot empowers your main burst even more, eyeball colector is the best one from this line, and then from the last line I prefer tresure hunter but ultimate hunter also works well.
The secondary runes are very important but they can change a lot if you prefer other options, i think triumph and alacrity since it gives you more dps and more power on 1v2s because of the extra healing, but magic footwear and future's market is very good as well. You could also use demolish and boneplating for better trades and unlimited splitpush potential.

Passive: Rek'sai's passive is very simple atacks generate fury and when you borrow you heal for a the fury you have.

Q: Now the Q is where she gets interasting, your Q is your main DPS tool and also your main Wave clear tool, be careful how many minions are around you if you are trying to freeze, this ability can break your own freezes and push to fast if you are not careful.
Q2: The Prey seeker is your ranged poke and can be used to farm at a distance if you are scared to get ganked or if your opponent is beating you down too much. But This ability is most usefull to scout areas where you expect someone to be in a bush or over a wall.

W: Your W changes your stances and burrows you, when burrowed you heal based on your fury and your abilites change into their second forms. While she is burrowed she has no vision of anything, but she can sense moving enemies and minions on a wide area.
note that if an enemy is standing still you cannot sense them and you might be walking into a trap. So scout with your Q2.
W2: She unburrowes and and knocks up one nearby enemy and briefly displaces every other enemy near her. The knock up as a coldown per enemy so time it well.

E: Your E is your biggest damage source, it is a point and click ability where rek'sai bites someone and deals a large amount of damage when rek'sai has full fury, never use this ability without full fury, you will miss out on a ton of damage.
E2: Is your tunnel wich allows you to slowly dash a fixed distance and leaves behind a tunnel which can be reused many many times for a slitly faster dash. Be very smart with your tunnel placements you can use it to roam from different angles, you can use it to get faster to lane, you can set up tunnels to easly get away from ganks and roams, and you can also use it as bait. Enemies will always try to destroy your tunnels, you can use this to get them to try and destroy it while you retake your tunnel and knock them up while they are not expecting.
R: Your R is a very stong ability, its a very good gap closer if you use it with your Q2, and it can also be used to dodge cc and abilities since you go untargetable while it casts. you can also negate some damage like zed's ult if you time it just right. You can only ult a marked target so never use your Q2 too early because if they get away you might need to mark them again for the kill.

I think the items,and how to itemise is well explained in the build notes. So check that for more info on builds.

Rek'sai paired with good macro and helping your team, you can climb really well this build works wonders to get out of low elo.