After a myriad of complaints about Yuumi I finally hit M5 (and now 6), and my win rate with her is steadily climbing to 60%, so I figured why not share what I use to the maybe 2 people who will care lol
I run **Glacial Augment** - Stopwatch, Future's Market and Approach Velocity (_Manaflow Band / Transcendence_)
Boots of Lucidity - Spellthief's - Twin Shadows - Hourglass - Mejai's - free space
(alot of talking if you care about that)
I love Glacial on Yuumi, it's saved me/my laner on more than one occasion, even slowing down a WW chasing my 40hp Sivir enough for her to kill him. Being able to pop out of W and surprise slow an enemy is incredibly useful and it feels like adding more power to her passive. Stopwatch (o.p.) is a godsend for Yuumi's who accidentally take turret aggro because you probably won't even notice you did, and can buy you those precious few seconds an ally needs to get close enough to the wall for you to dash to them. Future's Market is every support's tool for quick itemization and Approach Velocity lets you tether faster in situations where you really need to.
The plan for Yuumi is to not die as much as possible. I'm not going to go into a rant about playstyle, but if you play right your first back should let you buy Boots, gold upgrade and Dark Seal.
your 2nd item depends on whats going on in the game.
Are you super ahead with 6 stacks on Seal. Rush Mejais.
Do you have 2-3 right-clickers on your team. Probably get Censer if your carry is good with it.
Otherwise just upgrade boots and get Twin Shadows for Glacial to power up.
From there just focus on getting Mejais stacks, build Ardent or Redemption if your team needs them, but i try to avoid getting more than 1 **hard** support item. if you were forced to use Stopwatch consider buying Hourglass, but ignore it if you're not in danger of getting popped.
I don't like getting Athene's because I use her E charges too frequently to build a big enough count to make the item matter, I do however get a Mikael's Crucible when I absolutely need to remove a stun from my carry. This is even more important for Yuumi because if her carry is stunned, then Yuumi isn't going anywhere either.
I've also used Shurelya's and even Knight's Vow as a kind of "_buff adc as hard as possible_" build but that's very rare