Abilities of Yuumi
Spell skill priority is an important winning factor for Yuumi. Understanding the abilities of Yuumi in League of Legends is another. Below you can find all the relevant information on Yuumi abilities. In order for item builds you should check the build for Yuumi.
Passive: Feline Friendship
Periodically, when Yuumi strikes a champion with an attack or ability, she restores health to herself and the next ally she Attaches to.
While Attached, Yuumi generates a special bond with her allies. The ally with the strongest bond enhances Yuumi's abilities while she is Attached to them.
Prowling Projectile
Yuumi fires a missile, dealing damage and slowing first target hit. It deals bonus damage and an enchanced slow if it takes at least 1.35 seconds to get to its target. While on her Best Friend, the slow is always enhanced and grants bonus On-Hit damage to her ally.While Attached, the missile can be controlled with your cursor for a brief period.
Yuumi summons an errant missile that deals totalmissiledamage magic damage to the first enemy and Slows the target for slowamount%.
If cast while Attached, Yuumi can control the missile using her mouse for a short period before it accelerates in a straight line. The accelerated missile deals totalmissiledamageempowered magic damage and Slows the target for empoweredslowamount% for empoweredslowduration seconds, instead.
Best Friend Bonus: Prowling Projectile's Slow will always be enhanced and hitting an enemy champion also grants them onhitdamagecalc magic damage buffduration seconds.
The bonus On-Hit damage can be increased by allycritchancemaxamp*100% based her ally's Critical Strike Chance.
You and Me!
Yuumi dashes to a target ally, becoming untargetable from everything except turrets. While on her Best Friend, she gains Heal & Shield Power and grants her Ally On-Hit healing.
Passive: While on her Best Friend, Yuumi gains an additional healandshieldpower*100% Heal & Shield Power and her ally also restores healthonhit health
Active: Yuumi dashes to an ally champion and Attaches to them. While Yuumi is Attached, she follows her partner's movement and is Untargetable except from towers.
Immobilizing effects on Yuumi place this Ability on a ccattachlockout second Cooldown.
Zoomies
Shields Yuumi and boosts Move Speed and Attack Speed. If she's attached, she passes it to her ally instead.Yuumi shields herself, blocking totalshielding damage and gains totalattackspeed% Attack Speed for msduration seconds. While the shield persists, the target also gains msamount% Move Speed.
If Yuumi is Attached, this Ability affects her ally instead and also restores manarestore Mana to them, increased by up to maxmanapercincrease*100% based on the target's missing Mana.
Final Chapter
Yuumi channels five waves that damage enemies and heal allies. Yuumi can move, attach, and cast Zoomies! while channeling. While on her Best Friend, this spell also follows her mouse.
Yuumi channels for ultduration seconds, launching numberofwaves magical waves affecting both teams. If initially cast while Attached, Yuumi can steer the waves to follow her mouse.
Enemies struck are dealt totalmissiledamage magic damage and Slowed by baseslow*-100% for ccduration seconds, increased by bonusslowperwave*-100% per wave hit.
Ally champions are healed for totalhealperwave health per wave. Excess healing is converted to a Shield instead.
Best Friend Bonus: For her Best Friend, the heal is increased to enhancedhealperwave health.
Casting You and Me! will locks the waves in the curret direction.
Yuumi can move and cast Zoomies while channeling.
FAQ on Yuumi
What do you max first on Yuumi?
Most Yuumi pros max out 
Prowling Projectile first.