SORCERY

Pretty basic rune, always pick this because it provides shield to your teammates when you cast You and Me! or Zoomies.

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Very necessary for Yuumi because of the mana problems she has.

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An extra 10% cdr at level 10, better than the other two options.

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Works well with your Prowling Projectile and makes your poke stronger.

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INSPIRATION

ALWAYS pick this, if you don't have it, you will not be able to survive the laning phase. If you don't have enough mana and a trade/fight/gank is going to happen, pop one of these and it will be enough to help your teammates.

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Very useful and good combination with Biscuit Delivery if you have them both, then one biscuit will provide you enough mana to cast Zoomies. These two runes combined with good mana managment are really important.

PROS
* Very good peel
* Relaxing champion
* High utility
* Good engage/disengage with ultimate Final Chapter
* Strong healing

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CONS
* Relies almost completely on the rest of the team
* Can't keep up with offensive playstyles in the early game due to high mana costs
* Very squishy
* You and Me! can lead to a very dangerous situation if you dont use it correctly

Yuumi can be one of the best(if not the best) peelers in the game. If she chooses the correct person to peel, it can be really hard for the enemy team to deal with it. Most people think that because of the nerfs she is now useless, but i disagree with that. In order to play Yuumi after the nerfs, you have to understand what the nerfs did. She now has very high mana usage and her e Zoomies has a weird cost. Zoomies was way too powrful so the cost is different, it costs 40/45/50/55/60 mana PLUS +15% of her max mana. That means that your mana is very limited and you cannot cast you abilities uncontrollably (especially your Zoomies 0,in addition, mana items won't be that effective on her. In the early game you will not have mana regen items so you will want to use your Zoomies ONLY when necessary( something like soraka's heal which costs too much health in the early game and she can't spam it), don't just use your Zoomies when your adc takes damage, use it when you think it will make a difference in a trade, or when your adc is in danger. Other than that, you want to poke with your q Prowling Projectile while keeping an eye at your mana bar. Never use your Prowling Projectile if you will not have enough mana for Zoomies afterwards, as your adc might get into a tight situation and you will not be able to save him. Generally your early game must be passive and defensive, that's why Yuumi goes well with mid/late game champions like Ezreal.
Another tip i want to give you is about your ultimate Final Chapter. Don't get fooled by the visuals, the range of this ability is much shorter than it is actually shown. Drag your coursor over the skill to see its actual range. You might also want to turn smart cast off for this skill.

After the laning phase is over and after you build a couple of mana regen items, you are ready to rock and roll. Keep in mind that you have to peel the BEST player in your team NOT the one with the best K/D/A , don't let kills trick you. You want to peel the best late game champions, the best carry champions and the best players. For example if your team has a 9/2 Renekton and a 2/1 Veigar you will want to peel Veigar very hard, because he is an extremely strong champion in the late game while Renekton has a strong early game but falls off later on. The decision of who you will follow and peel is very important and you have to watch the playstyle and champion selection of your teammates so that you will make the best decision. You might also want to avoid players with a very offensive playstyle especially if you have stacks on your Mejai's Soulstealer. Don't forget that if the player that you follow dies, you will most probably die too, so you will give out 2 kills instead of 1.