Rek'sai a fitting name for a vicious (Female) creature from the void, and trust me she will rek you.

Ravenous Hydra paired with rek'sai's fury=heal means
more heal for you while not in a battle,
and you'll have a stalker's blade for jungling though it depends on which one you want.
A black cleaver for penetration health and damage,
A randuin's for escaping/sticking and more defense,
and now you pick one hybrid Item hextech or guinsoo's but you get it for the extra ability power so your Q while burrowed will do extra damage.
as for boots you'll have to decide.

Although flash is recommended you can have something else if you think you can handle it.
like
Exhaust,
Ignite or
Teleport

Pros
+High mobility
+Pretty Good CC
+Pretty Good Damage
+Pretty easy to learn
+Kawaii

Cons
-Countered by knock-ups
-Not Very Tanky
-Not Exactly a good ult

Go In this order
Grump
Wolves
Red
Rocks
Birds
Blue (Ask your allies if they want it)

Rek'sai in my opinion is good at starting team fights but your entire team has to come with you
or your gonna die so make sure everybody is gonna follow you in a team-fight.

Use your Q to build up fury and remember fury=health oh and when using your Q try to hit as many targets as possible cause you build up more rage the more targets you hit.

So that's pretty much all the information I know, thank you for viewing this build/guide AND PLEASE comment, I mean it comment comment now.