Hi, my name is Janno, I'm from Argentina. My nick in League of Legends (one of the 11 or 12 I had) is Gank us (yeah, I'm a Fence fan xD), server Latin America South. I started playing this game in end Season 3, but I started playing seriously last year. My peak elo was D3, being a full support player, but mostly with Yuumi.
Some time ago I was an OTP Thresh, one of the best in my server, but I didn't have any champ pool more than Thresh and Braum, so I tried some more supports, and Yuumi was the best for me, so I stopped using Thresh to become a Yuumi main.

Yuumi is a very unique champion, capable to attach to an ally, being inmune to every damage while being in it (except tower hits, if you deal damage even being attached). Personally I like the idea of Yuumi casting abilities even attached to his ally, controlling her Prowling Projectile by moving it as you want, and her Zoomies, a high heal, attack speed and movement speed buff, plus a lot of adaptive damage from her You and Me!.
Another good thing, but also a bad thing, is that she doesn't need escaping spells, so she can use Exhaust, Ignite or Heal, depending on which spell her ADC uses, Exhaust, Heal or Cleanse.
The bad thing of this, is that Yuumi on her own, is super hard to escape alive, as she, most of the cases, has to use her Final Chapter to root her enemies to try and escape alive.

Yuumi is an easy champion to learn how to use, but it's hard to use her properly.

A part of the laning phase you'll be attached to your ADC, detaching when your You and Me! cooldown is gone. It's important that you also make presence in lane and try to poke with your Prowling Projectile, trying to hit it empowered to deal more damage and to slow the enemy, making it harder for him to chase you and your ally that you're attached, or making it easier for your ally and you to chase her. Until Level 6, You and Me! has a 10 second cooldown when you instanly leave your ally, but it goes off even when you're attached.
Most of the times you should wait for the cooldown, go out of your ally to activate Feline Friendship, that gives extra mana and a shield, and attach again to your ally.
Level 6-11 your You and Me! cooldown is 5 seconds, and Level 11-18 has no cooldown, unless Yuumi gets CCed, that gives 5 seconds of cooldown on it anyways.

Most of the times, from mid to late game, you'll be attached to the hypercarry, giving him a massive healing and attack/movement speed, plus having your Final Chapter to iniciate teamfights aswell, or to trying to defend yourself and your ally. Try to proc Feline Friendship everytime you can, being careful of enemy CCs, for more mana and for a shield when you use You and Me! to go back to your ally.

Use Zoomies if necessary to give movement speed, for example, to dodge engage skillshots. Try not to always use it for movement speed, as this ability costs excesive mana. It costs 40-60 mana + 15% of your maximum mana, making this ability not so worth to max, since you'll probably have mana issues.

To end this guide, I want to thank every one that has read all the guide, it took some time for me to make it
This guide is being constantly updated, so, feel free to comment me if you see something wrong! I accept all kind of comments that can help me improve my guide
Have a good day/night and good luck in your games! <3
~Janno