Play lane with short off-on trades: detach only after the engage spell is gone, use one auto or eat one blockable projectile with your HP, then reattach before your W punish window opens
Yuumi Q needs a driver—cast it from a host whose movement gives the missile time to arm and steer, because the real slow comes from travel time and gets better on your Best Friend
Treat Best Friend as a lane bet and keep the game attachable: Yuumi gets less from early off-lane roam kills than peer enchanters, but one of the biggest long-game lifts in the role if you stay on the seat that can still carry later fights
Leave when your ADC is no longer the best seat for the next few fights, not just because someone else looks stronger on the scoreboard. If your marksman is still farming, still relevant in front-to-back fights, and still gives you good Q and Best Friend value, stay. Move when another teammate has clearly become the only body that can start or finish fights, or when bot lane is so far gone that sitting there only strands your kit. The easy check is simple: who will matter in the next 30 seconds, and who can actually use your shielding, speed, slow, and ult waves without immediately dropping you both into a lost position?
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.
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 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.
Shields Yuumi and boosts Move Speed and Attack Speed. If she's attached, she passes it to her ally instead.
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.